Check
Seems someone has a similar problem when cooking.
The user Light_Shock mentions
“I think that this is a bug in one of the blueprints that ships with Unreal. I changed the levels to cook to only those in my game (thereby excluding first person map etc.) and the issue cleared. The settings are in Projects \ packaging \ list of maps to include in packaged build.”
This is mainly in regard to the error "Mount point: ‘root:/’ is not mounted to a valid Root Path "