Rider shows lots of errors when opening an Unreal 5 project (but it sill builds)

I’m guessing you installed vanilla Rider that doesn’t have the Unreal plugin installed or updated.

Make sure you have the UnrealLink Rider plugin installed, then install the RiderLink plugin in your game.

Also check in Languages > C++ > Unreal Engine and make sure you have Enable UnrealHeaderTool inspections checked. (There are some other great options to check in there as well).

Good luck, hope this helps.