Removing pool 'mon_allow_pool_delete config option to true before you can destroy a pool1_U (500)

Another approach:

ceph tell mon.\* injectargs '--mon-allow-pool-delete=true'
ceph osd pool rm test-pool test-pool --yes-i-really-really-mean-it

You can set the config via the CLI or via the dashboard of Ceph under Cluster -> Configuration (advanced settings).

The CLI command is the following:

ceph config set mon mon_allow_pool_delete true