Even though you imported the php db, the converter asks you for phpbb path and reads a configuration file from it, in which most probably it's the old phpbb database pointed to. So the converter will try connecting to that one still, for phpbb data.
You can:
Option 1: - install SMF in the same database where phpbb was installed (not the other way around)
Option 2: - let them in different databases, but check in your host's panel if you can add the user of the SMF database to the phpbb database.
Option 3: - change the phpbb configuration file to point to the SMF database (where you imported the phpbb tables) - I don't recommend this, because your phpbb will try to work then from this copied tables, and if any is missing or whatnot, phpbb installation won't work. Rather, make a copy of the config file, and place it in another folder (random folder) and point the converter to that folder as "phpbb installation".