Did you run the command as it suggests?
Have you created a case with Tableau Support?
Sometimes the registration process takes a few goes to correctly apply, I think this is a known issue.
I suggest you try to re-register and see if it works on second attempt.
Yes Peter. It did worked. I have re-entered the key
Re-registering should be fine. We do not need to create a tableau Support ticket for this issue. It worked for me.