There is an unnecessary and confusing step in the solution below, which warrants a correction. Namely: there is no Preferences option on iPhone for the messages APP Here is the correct process: (this is for BIG SUR ONLY, though the instructions are for iOS 14) On your iPhone: 1) Go to Settings - Messages APP 2) Uncheck iMessage - this automatically unchecks what is called Send & Receive, NOT "Forward Messages to my Mac." 3) Then, recheck it back to Green This technically resets the synch between the iPhone and your recently updated machine to Big Sur, making the steps below unnecessary. ************ 2) Sign out of your Apple ID account 3) Sign back in with your Apple ID & password 4) Go back to Settings - Messages app - Preferences - enable iMessage on icloud and Forward Messages again. 5) Rebooted Mac...SUCCESS!