Does the _SYS_REPO user has select, execute & update privilege with grant option to the schema where the underlying tables used in decision tables belong to?
grant select on schema "<your_schema>" to _SYS_REPO with grant option
grant execute on schema "<your_schema>" to _SYS_REPO with grant option
grant update on schema "<your_schema>" to _SYS_REPO with grant option
This should solve the problem with missing authorization