works. I've done no createdb commands, so dopdb shouldn't be what I want for this cleanup (I'd think).
If you copied+pasted the exact command you run, then no wonder it is wrong. If you look closer you are not calling "--stop", the first dash is another character (not minus/hyphen character). One is "EN DASH" with code 0xe28093 [1], while the correct one is "HYPHEN-MINUS" with code 0x2d [2].
Also, execute pg_lsclusters to see what clusters you actually have on this instance, then you can use the version and cluster name provided there to call pg_dropcluster.