If your license number is not accepted (AVG says it is not valid), please check that it is valid. You can use this web form to get your license number be resent to your mailbox. Then, you can re-activate AVG using this license number.
However, if AVG accepts the license number and later it reports the number is invalid, the problem is caused by something else. Basic information on AVG license trouble can be found in this article.
I recommend creating new discussions in future, rather than resurrecting old threads. The forum is then friendlier to read for other visitors who may be facing the same problem.