HI @Unreality ,
The source code is currently broken with several errors.
This is very common there is a note somewhere saying that this is a problem.
I expect you have pulled source at a transient time when ue5nain was broken.
If you want to try my unofficial version which works, just to test your system
UnrealEngine-5/README.md at LGPU54bin-patch · jimshalo10/UnrealEngine-5 (github.com)
The main quote from README is here
Additional Notes
The first time you start the editor from a fresh source build, you may experience long load times. The engine is optimizing content for your platform and storing it in the derived data cache.
Obviously my version does NOT have Zen Server enabled as its EXPERIMENTAL