What are you trying to add? If you are modifying to add/remove some field from the forum, you need to understand how phpBB works in the coding background.
It is denying your access to the database. check that your username and password are correct. Your provider might be doing a maintenance on your server
The database complain about a duplicate entry. You might want to consider removing the SQL id 29973. Please do a backup of the database before you do anything.