If you use pycreateuserpkg to create a user or change its password, that will work great... except with Printopia Pro, which will prompt for a local admin username and password, and your local admin username and password will not work, and you'll get a login not accepted error message instead.
Deleting and re-adding the local admin account using the GUI (System Preferences > Users & Groups). You'll have to use the terminal to delete the user and then re-add (through the GUI is fine for this second part).
Then Printopia Pro will launch up just fine and not prompt you for a username or password.