This issue can be solved by making the Case Owner field blank in Case Settings.

Case Owner field is used to find the owner of the case, which can be either an individual user or a queue. This field is optional.


Note:
1. If you specify a case owner, auto-assignment rules are ignored.
2. You can’t delete a queue that a routing address refers to. Either delete the routing address, or edit the routing address settings to point to a different queue.