It’s not telling you that the header is inaccessible, but that the individual member in question is. Many previously-public members in 4.19 have been made private in 4.20 and exposed through accessors. I’m not in a position to check this particular case here, but when updating to 4.20, I did have to make changes like the following to my own classes:
StaticMeshComp->bGenerateOverlapEvents = false;
to
StaticMeshComp->SetGenerateOverlapEvents(false);
It’s likely that you’re in a similar situation and need to replace direct references to these member variables with calls to accessor functions.