The first time you backed up to a computer with Encrypt Backup checked it prompted for a password - twice. It then applied that password to every backup since that first one, even if years have passed since that backup, unless you specifically changed it. If you have a company email account on your phone backup encryption may have been required by the administrator, but you would still have had to enter the encryption key (password). It’s possible to make a new backup and change the encryption key. See this Apple support article—>About encrypted backups on your iPhone, iPad, or iPod touch - Apple Support, specifically the section if you can’t remember the encryption password.
If you you enabled Keychain on your device when you entered the passcode the first time there was an option to save it in Keychain, so you can find it there.
The other option is to guess it. If you are like most people it is a password that you have used elsewhere. You get unlimited guesses, and to speed up the process uncheck Encrypt Backup or Change Password.