The error message is quite self-explanatory. If you have any other processes using the DB – iex, mix phx.server or even psql itself – then you are not allowed to drop it.
In your case, since it’s only one other client using the DB, I’d bet on you having a psql shell opened somewhere. (When it’s iex or mix phx.server using the DB, you’ll see another number, usually 10.)
