WP Backup error

The “master password” is feature from FileZilla to encrypt passwords you saved in the configuration. It’s tied to a particular FileZilla installation, and something you can choose to enter when you run FileZilla for the first time.

So to answer your question on where to find that password, that would be in your memory or some other place where you typically store passwords. If you don’t have it anymore, I would assume FileZilla has a button to set a new password and forget all passwords currently stored.