Find the following:
C:\Users.…\AppData\Roaming\SketchUp\SketchUp 2018\SketchUp\Materials
Instead of deleting rename the folder so SU does not see it.
Find the following:
C:\Users.…\AppData\Roaming\SketchUp\SketchUp 2018\SketchUp\Materials
Instead of deleting rename the folder so SU does not see it.