TWRP: vendor partition is not existed I try to flash LineageOS on a OnePlus 5 (cheeseburger).  I installed TWRP 3.3.0-0 from here. I repeatedly wiped the partitions.

What I cannot manage is to adb sideload LineageOS 16.0.  I get this message:

Starting ADB sideload feature...
Installing zip file '/sideload/package.zip'
Error: Vendor partition doesn't exist!
Updater process ended with ERROR: 7


I have read here that I should install the latest stock ROM (from here), but that results in almost the same error message:

Starting ADB sideload feature...
Installing zip file '/sideload/package.zip'
vendor partition is not existed, exit ota!!
Updater process ended with ERROR: 7


How can I create/populate/fix that vendor partition?
OnePlus 5 didn't support project Treble out of the box, so there was no /vendor partition. However with update to Android P (as explained here), said partition was created to support Treble. If you didn't undergo that OTA update, you must haven't /vendor partition on your device.

You can also create the partition yourself from within TWRP if you are a bit tech savvy. parted or gdisk can be used for the purpose. But playing with partition table could be risky if you aren't sure what you are doing. You can see the partition table of your device in given link.