1) check runas of your workflow as job will consider user/sp defined in runas of your workflow. This should have all necessary permissions.

2) if this table is already added to catalog.schema.table in UC then this user/sp should have necessary permissions there as well as I have seen this supercede external location permissions.

3) if point 2 is not valid then provide read file and write file to this user/sp on external location in UC.

give a try..