You have lost your file. VocaDial cannot rebuild it: it has never had your first names. Enter them again from your printed attendance sheet (“Students” window of a tool, one first name per line), or recreate the codes with your original list (“Recreate the codes with a list” in “Students and slips”).
Your file asks for a passphrase. That is the old format. Open it one last time with its passphrase: the file VocaDial then gives you is protected by your account, and you will have no passphrase to remember.
You have forgotten the passphrase. It cannot be recovered, by anyone. Enter your first names again as above: the new file will have no passphrase.