@David Riedy 

What training board are you referring to? Perhaps the person who set up the training board will know the password of the board that is being linked to. 

If not, as the board owner (and co-owner) are the only ones who can set a password on a public/visitor access board, maybe the person who set out the training board will know the owner of the password protected board and can either get the password or they can reset it and share the new password. 

Talking about the Miro Academy training. The first training board after the first lesson asked for a password. Subsequent training boards in different lessons were fine.

Do you remember which course this happened in?

https://academy.miro.com/