[HTC U12+ ROMs, Kernels, Recoveries, & Other Develo] st_voss:
Got the following error when installing Magisk via TWRP

Installing zip file '/external_sd/download/Magisk-v16.0.zip'
Checking for Digest file...
Skipping Digest check: no Digest file found
I:Update binary zip
Verifying package compatibility...
Package doesn't contain compatibility.zip entry
I:Zip does not contain SELinux file_contexts file in its root.
I:Legacy property environment not used in updater.
Archive: /external_sd/download/Magisk-v16.0.zip
inflating: META-INF/com/google/android/update-binary
inflating: META-INF/com/google/android/updater-script
inflating: addon.d/99-magisk.sh
inflating: arm/magiskboot
inflating: arm/magiskinit
inflating: arm64/magiskboot
inflating: arm64/magiskinit
inflating: chromeos/futility
inflating: chromeos/kernel.keyblock
inflating: chromeos/kernel_data_key.vbprivk
inflating: common/boot_patch.sh
inflating: common/magisk.apk
inflating: common/util_functions.sh
inflating: x64/magiskboot
inflating: x64/magiskinit
inflating: x86/magiskboot
inflating: x86/magiskinit
inflating: META-INF/MANIFEST.MF
inflating: META-INF/CERT.SF
inflating: META-INF/CERT.RSA
************************
* Magisk v16.0 Installer
************************
- A/B partition detected, current slot: _b
- Mounting /system, /vendor
mount: mounting /dev/block/platform/soc/1d84000.ufshc/by-name/system_b on /system failed: No such file or directory
! Cannot mount /system
- Unmounting partitions
Updater process ended with ERROR: 1
I:Install took 1 second(s).
Error installing zip file '/external_sd/download/Magisk-v16.0.zip'
Updating partition details...

So TWRP cannot mount the system partition.

Found that /system is a symbolic link to /system_root/system.

But the directory /system_root/system that is used as mount point for /system is missing.

So one has to create /system_root/system. E.g.

adb shell
mkdir /system_root/system

When /system_root/system is present TWRP can mount the system partition and installing Magisk works.

Installing zip file '/external_sd/download/Magisk-v16.0.zip'
Checking for Digest file...
Skipping Digest check: no Digest file found
I:Update binary zip
Verifying package compatibility...
Package doesn't contain compatibility.zip entry
I:Zip does not contain SELinux file_contexts file in its root.
I:Legacy property environment not used in updater.
Archive: /external_sd/download/Magisk-v16.0.zip
inflating: META-INF/com/google/android/update-binary
inflating: META-INF/com/google/android/updater-script
inflating: addon.d/99-magisk.sh
inflating: arm/magiskboot
inflating: arm/magiskinit
inflating: arm64/magiskboot
inflating: arm64/magiskinit
inflating: chromeos/futility
inflating: chromeos/kernel.keyblock
inflating: chromeos/kernel_data_key.vbprivk
inflating: common/boot_patch.sh
inflating: common/magisk.apk
inflating: common/util_functions.sh
inflating: x64/magiskboot
inflating: x64/magiskinit
inflating: x86/magiskboot
inflating: x86/magiskinit
inflating: META-INF/MANIFEST.MF
inflating: META-INF/CERT.SF
inflating: META-INF/CERT.RSA
************************
* Magisk v16.0 Installer
************************
- A/B partition detected, current slot: _b
- Mounting /system, /vendor
- Device skip_initramfs detected
- Found boot/ramdisk image: /dev/block/sde34
- Found dtbo image: /dev/block/sde42
- Device platform: arm64
- Constructing environment
libc: Access denied finding property "persist.vendor.dpm.feature"
libc: Access denied finding property "persist.vendor.dpm.feature"
Failed to initialize runtime (check log for details)
libc: Access denied finding property "persist.vendor.dpm.feature"
libc: Access denied finding property "persist.vendor.dpm.feature"
- Encrypted data detected, keep forceencrypt
- Unpacking boot image
libc: Access denied finding property "persist.vendor.dpm.feature"
libc: Access denied finding property "persist.vendor.dpm.feature"
MagiskBoot v16.0(1600) (by topjohnwu) - Boot Image Modification Tool
Parsing boot image: [/dev/block/sde34]
KERNEL [16864562]
RAMDISK [5745661]
SECOND [0]
EXTRA [0]
PAGESIZE [4096]
OS_VERSION [8.0.0]
PATCH_LEVEL [2018-06]
NAME [boot:0]
CMDLINE [video=vfb:640x400,bpp=32,memsize=3072000 ehci-hcd.park=3 lpm_levels.sleep_disabled=1 service_locator.enable=1 swiotlb=2048 androidboot.configfs=true androidboot.usbco
ntroller=a600000.dwc3 printk.devkmsg=on androidboot.hardware=htc_ime androidkey.dummy=1 buildvariant=user]
CHECKSUM [c623de95f90cff7fcba3b0f9be62bdef5f752268]
DTB [2641261]
KERNEL_FMT [lz4]
RAMDISK_FMT [gzip]
- Checking ramdisk status
libc: Access denied finding property "persist.vendor.dpm.feature"
libc: Access denied finding property "persist.vendor.dpm.feature"
MagiskBoot v16.0(1600) (by topjohnwu) - Boot Image Modification Tool
Loading cpio: [ramdisk.cpio]
- Stock boot image detected
- Backing up stock boot image
libc: Access denied finding property "persist.vendor.dpm.feature"
libc: Access denied finding property "persist.vendor.dpm.feature"
MagiskBoot v16.0(1600) (by topjohnwu) - Boot Image Modification Tool
Compressing to [stock_boot_90606e2c0bcadc1f8970a098fe1bb15039380297.img.gz]
- Patching ramdisk
libc: Access denied finding property "persist.vendor.dpm.feature"
libc: Access denied finding property "persist.vendor.dpm.feature"
MagiskBoot v16.0(1600) (by topjohnwu) - Boot Image Modification Tool
Loading cpio: [ramdisk.cpio]
Add entry [init] (0750)
Patch with flag KEEPVERITY=[true] KEEPFORCEENCRYPT=[true]
Save SHA1: [90606e2c0bcadc1f8970a098fe1bb15039380297] -> [.backup/.sha1]
Loading cpio: [ramdisk.cpio.orig]
Backup mismatch entry: [init] -> [.backup/init]
Dump cpio: [ramdisk.cpio]
libc: Access denied finding property "persist.vendor.dpm.feature"
libc: Access denied finding property "persist.vendor.dpm.feature"
MagiskBoot v16.0(1600) (by topjohnwu) - Boot Image Modification Tool
libc: Access denied finding property "persist.vendor.dpm.feature"
libc: Access denied finding property "persist.vendor.dpm.feature"
MagiskBoot v16.0(1600) (by topjohnwu) - Boot Image Modification Tool
Patch @ 01B2410A [736B69705F696E697472616D6673]->[77616E745F696E697472616D6673]
- Repacking boot image
libc: Access denied finding property "persist.vendor.dpm.feature"
libc: Access denied finding property "persist.vendor.dpm.feature"
MagiskBoot v16.0(1600) (by topjohnwu) - Boot Image Modification Tool
Parsing boot image: [/dev/block/sde34]
KERNEL [16864562]
RAMDISK [5745661]
SECOND [0]
EXTRA [0]
PAGESIZE [4096]
OS_VERSION [8.0.0]
PATCH_LEVEL [2018-06]
NAME [boot:0]
CMDLINE [video=vfb:640x400,bpp=32,memsize=3072000 ehci-hcd.park=3 lpm_levels.sleep_disabled=1 service_locator.enable=1 swiotlb=2048 androidboot.configfs=true androidboot.usbco
ntroller=a600000.dwc3 printk.devkmsg=on androidboot.hardware=htc_ime androidkey.dummy=1 buildvariant=user]
CHECKSUM [c623de95f90cff7fcba3b0f9be62bdef5f752268]
DTB [2641261]
KERNEL_FMT [lz4]
RAMDISK_FMT [gzip]
Repack to boot image: [new-boot.img]
KERNEL [16860206]
RAMDISK [6116122]
SECOND [0]
EXTRA [0]
PAGESIZE [4096]
OS_VERSION [8.0.0]
PATCH_LEVEL [2018-06]
NAME [boot:0]
CMDLINE [video=vfb:640x400,bpp=32,memsize=3072000 ehci-hcd.park=3 lpm_levels.sleep_disabled=1 service_locator.enable=1 swiotlb=2048 androidboot.configfs=true androidboot.usbco
ntroller=a600000.dwc3 printk.devkmsg=on androidboot.hardware=htc_ime androidkey.dummy=1 buildvariant=user]
CHECKSUM [26c716e4a237d161983faaa689fb09a454e9e4bc]
libc: Access denied finding property "persist.vendor.dpm.feature"
libc: Access denied finding property "persist.vendor.dpm.feature"
MagiskBoot v16.0(1600) (by topjohnwu) - Boot Image Modification Tool
Cleaning up...
- Flashing new boot image
- Unmounting partitions
- Done
I:Updater process ended with RC=0
I:Install took 11 second(s).
Updating partition details...

TWRP-Version: TWRP_3.2.2-1_Imagine_Release_1.21

Firmware: 1.21.401.3 DUAL-SIM