In database jargon, a relation, in simplest terms, refers to a table or a view.
In Panoply, the error
must be owner of relation X — where
X is any table name — means you don't own the table or view you're trying to modify or drop/remove (manually, using SQL). Modifying or dropping/removing a table is possible only if you are its owner.
The tables that were created via Panoply's data sources are owned by the account owner. Even if a user is a team member under someone else's account and creates and ran data sources under it, the tables created will still belong to the account owner. The surest way a team member can own a table or view is to create them manually using SQL.
If you encounter this error and still need to modify or drop a relation, contact email@example.com.