Password Isn't accepted for certain security related functions on new iMac

My new iMac with Big Sur 11.1 won't accept my password to make certain changes, such as to Security & Privacy in System Preferences. It's an administrator account & it works to login and for certain other things like keychain access. It works on the MacBook Pro (High Sierra 10.13.6, too old for newer OS) that I transferred to the iMac.