This is a user to user forum. We can't help you with this issue.
If you're having problems with the serial number, you'll need to contact Adobe Customer Service. Or, if you bought your program used or in an open box, you'll need to contact the retailer who sold you an invalid serial number.
True, Adobe is the only one who can correct product activation issues.
And, in that regard, you could try to reach out to Adobe via its Adobe Chat. You would click on the following link and, when it opens, click on the statement "Still Need Help? Contact Us." to bring up Adobe Chat to discuss "Serial Number Issues or Activating My Product"
But I would ask you these questions for you to carry with you
a. Are you installing the program from a User Account with Administrative Privileges - Windows or Mac?
b. Do you have the latest version of QuickTime installed
c. Are you using a "redemption code" or the actual "serial number"
Please let us know the outcome.