Please refer the article: http://helpx.adobe.com/creative-suite/kb/error-serial-number-valid-product.html and contact our chat support in case of any further issue.
I tried the method of trying it as a trial version and I still couldn’t get the serial number to work. I am trying to open photoshop and the Master collection serial key pops up and I try the serial number and it will not work. Do I need to uninstall and reinstall and then put that I have the serial number instead of putting it as trial version? I have already deactivated it from the other computer. I greatly appreciate your support.