I just got this problem coming with my Mac on Sierra and my iPhone on iOS 10.
I ve deleted the text forwarding message toggle accidently and couldn't receive anymore any text into the MacBook Pro.
The solution for me was to logout of the iMessage icloud account under settings > message > Send and receive > Click on the Apple ID > sign out.
Sign in with another Apple ID into the message section > send and receive > sign in > activated
Go to the Mac and sign out of iMessage going to messages > option top left corner > Apple ID sign out > sign in with the secondary Apple ID >
The Mac will receive the code and will be associated whenever you change the Apple ID since this feature is related to text message and your phone number actually havent changed.
So now you just want to disconnect again from the iPhone first and then connect again >go to the Mac and do the same >
Then you'll have the message iMessage is now used on your new iPhone or Mac
close the settings app on the iPhone
Then you should be able to send text messages again
I hope I ve been clear enough