Thanks for reaching out to the  Apple Support Communities! It sounds like you have forgotten the encryption password for your iTunes backup. I know how important it is to be sure you are able to access iTunes backups. Check out this article here About encrypted backups in iTunes. Specifically: 
 

If you can’t remember the password for my encrypted backup

You can't use an encrypted backup if you don't have the password. 

When you try to use a backup, you might see a message saying "The password you entered to unlock your backup was incorrect." This message means that the iTunes encryption option was turned on in the past by:

• Yourself
• Someone who set up your device or transferred your data to a new device
• An installed profile (for business or education devices). If you're using an installed profile, contact your system administrator.

If you don't have your password:

• If someone else set up your device, check with them for the password. 
• Try using an iCloud backup instead. If you don't have an iCloud backup, you can make one.
• If it's still relevant, try using an older iTunes backup.