I managed to flash Lineage 18.1/Android 11, but there was one extra step needed in addition to the above! And I already tried flashing vendor.img from the stock rom without success too.EDIT: Funny you should reply when I hit a stroke of genius... I wrote this before seeing your reply, @TiTiB!
Eureka! The OP's guide is both missing steps and poorly written which led to even a senior member @TiTiB giving up on this. Fortunately, I just figured out a repeatable, working method for installing and I just got LineageOS installed. It is so easy, actually!
Here are the tools you will need to gather!
Minimal ADB and Fastboot 1.4.3
"vbmeta.img" from Android 10 GSI available here: developer .android .com/topic/generic-system-image/releases
Your GSI of choice (I used LineageOS). Extract the .img file and rename it to "system.img". You will need this later!
Now for the main steps you will need to follow!
1) Install Minimal ADB and Fastboot 1.4.3.
2) Copy "vbmeta.img" and "system.img" to "C:\Program Files (x86)\Minimal ADB and Fastboot".
3) Enable developer mode on your Ouketel K9. You do this, while in the Settings app, going to About and tapping on Build number 7 times.
4) While in the Settings app on your Ouketel K9, go to System->Advanced->Developer Options and enable both "OEM unlocking" and "Enable USB Debugging."
5) Now connect your Ouketel K9 to your computer via USB cable. Then get it into the bootloader. You do this, while in a powered off state, by holding volume up and power button, and choosing Fastboot mode.
6) From your computer's start menu, run Minimal ADB and Fastboot as administrator.
7) Run the following commands from the command prompt window that just appeared:
> fastboot devices
> fastboot flashing unlock
> fastboot --disable-verity --disable-verification flash vbmeta vbmeta.img
> fastboot erase system
> fastboot flash system system.img
> fastboot -w
> fastboot reboot
You may have noticed "fastboot flashing unlock." This command is very necessary and often overlooked! @Levered's obscure references to "already unlocked bootloader" and "Please unlocked your bootloader, if that" refer to this command. He didn't realize he didn't actually tell us this command. I literally just stumbled on it here:
www. getdroidtips. com/flash-twrp-recovery-android/#comment-131543
IMPORTANT: if it automatically goes into TWRP recovery (= fail!) after running fastboot reboot (step 8) then you need to click (in TWRP) on Advanced > ADB Sideload and run this command from your PC: adb sideload Magisk-v21.4.zip (and gapps if you want it - pico is usually best: https://sourceforge.net/projects/opengapps/files/arm64/test/20210130)
Note: there are reports of the most recent two versions of Magisk not working properly at the time of writing, so I used 21.4 as the recommended version.
Note also: gapps needs to be installed ahead of Magisk, and you may need to resize system partition first by following the Format option followed by the Repair File System option before going back to ADB Sideload:
Got error 70 while installing Gapps in TWRP? This error prompts up when your partition storage size is lesser than the size of the respective Gapps package.
The bootloop issue and the need for Magisk (normally used for rooting) is, apparently, due to something called "dm verity" being enabled by default on all phone's stock boot images:
FYI - here's where I got the main files:
1) vbmeta from Android 11 generic system image:
BTW, I don't know how well the GSI rom will work with this phone yet - haven't tested - but LineageOS 18.1/Android 11 is a Godsend when it comes to using a connected mouse, native "Files" app, freeform windows and the Taskbar 6.0 app as part of a desktop experience. Android 9 just didn't make the cut, hence freeform windows were not officially supported that time nor before, and caused apps to constantly crash. I think Android 10 might be somewhere in the middle, as I need to test that too.