I’ve seen in posts from 2020 that PROS used to support up to LVGL 5, but that’s hoplessly outdated. (So much that you can’t even visit the LVGL 5 docs anymore). What version does it currently support?
8.3.4 (make sure you’re using PROS 4)
Also LittlevGL documentation has an archive of the 5.3 docs if you really want to see them, but you should use 8.3