Thanks, @Kairion!
I was creating the database through the Plesk Panel.
The error has since resolved itself, and I suspect it was related to initialization of my account.
I didn't save the exact error, when I encountered it, but it said something to the effect that another user was currently using database (template1, iirc).
So for future reference, if you encounter this error as well, you may need to wait about an hour after creating your account before you will be able to create a postgres database.