When activating iMazing on my desktop, I am asked to type in my iphone's backup encryption password. I do not know what that password is. What do I do?

iMazing user here. Backup encryption password is not related to itself and won't affect activation or app features.

The encryption password is required to open the existing backup on your computer (made with iTunes or iMazing). If you still have the same backup encryption password on the device best you can do is to reset it according to the Apple guide, then change backup location in iMazing to a new folder and start a fresh backup set. In this way, you'll still have the old encrypted backup and maybe you'll be able to remember the password later.

Btw why haven't you contacted their sup? I usually get replies in 2-12 hours.

You cannot open a backup without the backup encryption password. You created the password the first time you made an encrypted backup, and you entered it twice. It never changes after it is created (unless you explicitly create it), and it is never prompted for again. If you have a Mac you were given the opportunity to save the password to Keychain, so if you have a Mac check there. If you haven’t erased the phone you can create a new encryption password—>About encrypted backups on your iPhone, iPad, or iPod touch - Apple Support, but other than that all you can do is guess. You get unlimited guesses, so think about what you might have entered as a password whenever that first encrypted backup was made.

IF you don't know your encrypted iTunes backup password, then you can't use it. The software is useless.