Hi @pjny-2020 and welcome to the Sound Forge group.
Which version of Sound Forge? 365 versions only allow activation to one PC at a time. Other versions are two... If that's the case, log into your account in the Magix Service Center to see and deactivate one,.. or a deactivation can be done in the SF "Help" menu
Use the email address that you used when the product was originally purchased. Otherwise contact the Magix Customer Service Sales Team which handles activation issues and such.