I had the same problem and found how to solve it.
I would leave info about this error for someone who opens this page in the future.
This error means that “Udon C# Program Asset” is missing, so you can solve this by deleting the corrupted asset or setting a script to the asset.
But where is that corrupted asset?
In my case, it is in a directory named “_UdonProgramSources”, and this directory is in a same directory with a scene file having a corrupted U# script component.
Maybe you can find that with clicking U# script’s Program Source.
Click “Sleep Station Udon C# Program Asset” in this screenshot, you will find Program Sources directory in assets window.
This directory has some Udon Program Asset files.
If you are in a correct Sources directory, you will find an asset which Source Script is None.
Delete that asset or set U# script, this error is banished and you don’t see that anymore.