If custom stuff is used and HLODs are built, the custom stuff is used and referenced by the HLOD.
When you delete the custom things from your project, sometimes the reference to that custom thing isn’t deleted from the HLOD file.
And when it tries to launch it can’t find the deleted file.
To fix this I totally delete the HLOD layer and start with a blank one.
But it has to be done in a certain order or it doesn’t work properly.
But this may not be the problem, so doing a Build Landscape will do more than build landscape it will check your map for common problems.