[OnePlus 7T Pro (Regular & McLaren) ROMs & Dev.] tzic:
I followed the link you sent. it's a bit complicated for me. I am not sure I understood correctly, but do I have to flash everything to both partitions?

these are the errors I get during the procedure which I follow by letter. Can you please take a look and advice on how I should proceed?

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot devices

XXXXXXXXX fastboot

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot -w

Creating filesystem with parameters:

Size: 232382812160

Block size: 4096

Blocks per group: 32768

Inodes per group: 8192

Inode size: 256

Journal blocks: 32768

Label:

Blocks: 56734085

Block groups: 1732

Reserved block group size: 1024

Created filesystem with 11/14188544 inodes and 938604/56734085 blocks

target reported max download size of 805306368 bytes

Erase successful, but not automatically formatting.

Can't determine partition type.

FAILED (remote: GetVar Variable Not found)

erasing 'userdata'...

OKAY [ 0.816s]

sending 'userdata' (152025 KB)...

OKAY [ 3.230s]

writing 'userdata'...

OKAY [ 0.006s]

erasing 'cache'...

FAILED (remote: Check device console.)

finished. total time: 7.506s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot erase system

erasing 'system'...

FAILED (remote: Check device console.)

finished. total time: 0.013s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot erase boot

erasing 'boot'...

OKAY [ 0.017s]

finished. total time: 0.018s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot erase odm

erasing 'odm'...

FAILED (remote: Check device console.)

finished. total time: 0.009s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot erase product

erasing 'product'...

FAILED (remote: Check device console.)

finished. total time: 0.010s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot erase vbmeta

erasing 'vbmeta'...

OKAY [ 0.005s]

finished. total time: 0.006s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot erase vbmeta_system

erasing 'vbmeta_system'...

OKAY [ 0.006s]

finished. total time: 0.007s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot erase vendor

erasing 'vendor'...

FAILED (remote: Check device console.)

finished. total time: 0.007s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot erase dtbo

erasing 'dtbo'...

OKAY [ 0.007s]

finished. total time: 0.008s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot flash system system.img

target reported max download size of 805306368 bytes

Invalid sparse file format at header magi

sending sparse 'system' (736725 KB)...

OKAY [ 16.810s]

writing 'system'...

FAILED (remote: Partition not found)

finished. total time: 16.825s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot flash boot boot.img

target reported max download size of 805306368 bytes

sending 'boot' (98304 KB)...

OKAY [ 2.095s]

writing 'boot'...

OKAY [ 0.313s]

finished. total time: 2.412s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot flash odm odm.img

target reported max download size of 805306368 bytes

sending 'odm' (1040 KB)...

OKAY [ 0.031s]

writing 'odm'...

FAILED (remote: (odm_a) No such partition)

finished. total time: 0.046s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot flash product product.img

target reported max download size of 805306368 bytes

Invalid sparse file format at header magi

sending sparse 'product' (759505 KB)...

OKAY [ 17.054s]

writing 'product'...

FAILED (remote: Partition not found)

finished. total time: 17.065s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot flash vbmeta vbmeta.img

target reported max download size of 805306368 bytes

sending 'vbmeta' (8 KB)...

OKAY [ 0.017s]

writing 'vbmeta'...

OKAY [ 0.006s]

finished. total time: 0.025s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot flash vbmeta_system vbmeta_system.img

target reported max download size of 805306368 bytes

sending 'vbmeta_system' (4 KB)...

OKAY [ 0.009s]

writing 'vbmeta_system'...

OKAY [ 0.009s]

finished. total time: 0.020s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot flash vendor vendor.img

target reported max download size of 805306368 bytes

Invalid sparse file format at header magi

sending sparse 'vendor' (755214 KB)...

OKAY [ 17.260s]

writing 'vendor'...

FAILED (remote: Partition not found)

finished. total time: 17.272s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot flash dtbo dtbo.img

target reported max download size of 805306368 bytes

sending 'dtbo' (16384 KB)...

OKAY [ 0.352s]

writing 'dtbo'...

OKAY [ 0.077s]

finished. total time: 0.432s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps> fastboot reboot

rebooting...

finished. total time: 0.003s

PS G:\tmp\platform-tools\Havoc-OS-v3.7-20200723-hotdog-Official-GApps>