[LeEco Le Max 2 ROMs, Kernels, Recoveries, & Other ] ThE_MarD:
Heyyo [MENTION=4280089]mlody95pl[/MENTION] , please ensure you are using either the latest official or test build of TWRP based on 3.3.0 Android 9.0 Pie. I just tested adb sideload and it works for me. Please also ensure adb and fastboot are updated on your PC.

Here's proof from my log of flashing firmware-20S-x829-le_x2.zip via adb sideload.

adb version
Android Debug Bridge version 1.0.39
Version 1:8.1.0+r23-5~18.04
Installed as /usr/lib/android-sdk/platform-tools/adb

fastboot --version
fastboot version 1:8.1.0+r23-5~18.04
Installed as /usr/lib/android-sdk/platform-tools/fastboot

I:operation_start: 'Sideload'
Starting ADB sideload feature...
E:[MTP] request read returned -1, errno: 5I:Disabling MTP
Stopping adbd...
adbd pid 497 found, sending kill.
BOARD_HAS_NO_REAL_SDCARD := true
RECOVERY_SDCARD_ON_DATA := true
I:Lun file '/sys/class/android_usb/android0/f_mass_storage/lun0/file' does not exist, USB storage mode disabled
TW_INCLUDE_CRYPTO := true
I:TW_BRIGHTNESS_PATH := /sys/class/leds/lcd-backlight/brightness
I:Found brightness file at '/sys/class/leds/lcd-backlight/brightness'
I:Got max brightness 255 from '/sys/class/leds/lcd-backlight/max_brightness'
I:TWFunc::Set_Brightness: Setting brightness control to 102
I:LANG: en
I:TWFunc::Set_Brightness: Setting brightness control to 5
I:TWFunc::Set_Brightness: Setting brightness control to 0
sideload-host file size 46378041 block size 65536
I:TWFunc::Set_Brightness: Setting brightness control to 25
Installing zip file '/sideload/package.zip'
unknown fuse request opcode 2016
I:Update binary zip
Verifying package compatibility...
Package doesn't contain compatibility.zip entry
I:Extracting updater binary 'META-INF/com/google/android/update-binary'
I:Zip does not contain SELinux file_contexts file in its root.
I:has_legacy_properties: Found legacy property match!
I:Legacy property environment initialized.
unknown fuse request opcode 2016
UFS device detected
Target: LeEco/LeMax2_NA/le_x2_na:6.0.1/FIXNAOP5802011301S/letv11301725:user/release-keysAttempting to process /dev/block/bootdevice/by-name/sbl1bak

/dev/block/bootdevice/by-name/sbl1bak not present. Skipping
Attempting to process /dev/block/bootdevice/by-name/rpmbak
Copying /dev/block/sde into lun_list[0]
Patching firmware images...Attempting to process /dev/block/bootdevice/by-name/tzbak

Attempting to process /dev/block/bootdevice/by-name/abootbak
Attempting to process /dev/block/bootdevice/by-name/hypbak
Attempting to process /dev/block/bootdevice/by-name/lksecappbak
/dev/block/bootdevice/by-name/lksecappbak not present. Skipping
Attempting to process /dev/block/bootdevice/by-name/keymasterbak
Attempting to process /dev/block/bootdevice/by-name/cmnlibbak
Attempting to process /dev/block/bootdevice/by-name/cmnlib64bak
Attempting to process /dev/block/bootdevice/by-name/pmicbak
Preparing /dev/block/sde for update stage 1
gpt_get_state: Block size is 4096
gpt_get_state: Block size is 4096
UFS device detected
Attempting to process /dev/block/bootdevice/by-name/sbl1bak
/dev/block/bootdevice/by-name/sbl1bak not present. Skipping
Attempting to process /dev/block/bootdevice/by-name/rpmbak
Copying /dev/block/sde into lun_list[0]
Attempting to process /dev/block/bootdevice/by-name/tzbak
Attempting to process /dev/block/bootdevice/by-name/abootbak
Attempting to process /dev/block/bootdevice/by-name/hypbak
Attempting to process /dev/block/bootdevice/by-name/lksecappbak
/dev/block/bootdevice/by-name/lksecappbak not present. Skipping
Attempting to process /dev/block/bootdevice/by-name/keymasterbak
Attempting to process /dev/block/bootdevice/by-name/cmnlibbak
Attempting to process /dev/block/bootdevice/by-name/cmnlib64bak
Attempting to process /dev/block/bootdevice/by-name/pmicbak
Preparing /dev/block/sde for update stage 2
gpt_get_state: Block size is 4096
gpt_get_state: Block size is 4096
UFS device detected
Attempting to process /dev/block/bootdevice/by-name/sbl1bak
/dev/block/bootdevice/by-name/sbl1bak not present. Skipping
Attempting to process /dev/block/bootdevice/by-name/rpmbak
Copying /dev/block/sde into lun_list[0]
Attempting to process /dev/block/bootdevice/by-name/tzbak
Attempting to process /dev/block/bootdevice/by-name/abootbak
Attempting to process /dev/block/bootdevice/by-name/hypbak
Attempting to process /dev/block/bootdevice/by-name/lksecappbak
/dev/block/bootdevice/by-name/lksecappbak not present. Skipping
Attempting to process /dev/block/bootdevice/by-name/keymasterbak
Attempting to process /dev/block/bootdevice/by-name/cmnlibbak
Attempting to process /dev/block/bootdevice/by-name/cmnlib64bak
Attempting to process /dev/block/bootdevice/by-name/pmicbak
Preparing /dev/block/sde for update stage 3
gpt_get_state: Block size is 4096
gpt_get_state: Block size is 4096
script succeeded: result was [1.000000]I:Updater process ended with RC=0
I:Legacy property environment disabled.
I:Install took 11 second(s).
I:Signaling child sideload process to exit.
I:Waiting for child sideload process to exit.
sideload_host finished