Refer to the cautionary information in Bucket Management: Deleting.
You cannot delete a bucket that has objects under retention. While you wait for the retention period to expire, you can set up a Lifecycle rule to start cleaning out the bucket by removing the objects that are no longer under retention. To set up a Lifecycle rule, refer to the procedure in Deleting a Large Bucket above.
If you are using a backup application, make sure all references to the bucket are removed from the application so that the retention period is not continually reset while the application communicates with the bucket.
If you are unsure of how long objects are under retention in the bucket, Wasabi Support created a Python script that can run through the bucket and determine the longest retention period of all objects.