There isn't any serial number to activate the Photoshop Photography Program.
Please follow the steps below regarding the issue resolution
1) Clean up cached user login information.
1. Close the Creative Cloud application.
2. Navigate to the OOBE folder.
Windows: [System drive]:\Users\[user name]\AppData\Local\Adobe\OOBE
Mac OS: /Users/[user name]/Library/Application Support/Adobe/OOBE
3. Delete the opm.db file.
4. Launch Creative Cloud.
2) Reset the hosts file
NOTE: You may have to disable you security software to modify your hosts file.
Mac OS Log in as an administrator to edit the hosts file in Mac OS.
Please let me know if you have any questions.