You are attempting to import custom object records using CSV file which worked just fine in Production but throwing warnings in non-production environment.
CSV Import Completed (with warnings)
'Your Attribute' : attribute(s) are not supported
You also confirmed that the attribute it is complaining about does exist in Production and it is Active.
16.2.3, 16.3.0
API Attribute ID did not match between non-production and production environment.
API Attribute ID needed to be updated to match non-production environment.