Most android ROMs need proprietary firmware file from the device's vendor partition to boot. These firmware files change from device to device, and also between android versions. The android version associated with the currently flashed vendor partition is your device's "vendor version". They are named either by the android version number ("9", "11"), or by the android version code ("P", "R"). The android version codes can be found at apilevels.com.

The custom ROMs usually only work on a specific vendor version (unless they are "Vendor OSS", which means they come with their own vendor partition).

You can change the vendor version by flashing the appropriate official ROM to your device. Note that most OEMs have their own version system, which does not track the Android version of their ROMs.