Hi,
There are no serial numbers associated with Adobe CC.
In case you still get the pop up for serial number, please refer to the help document below:
Creative Cloud applications ask for serial number
You may also try to check your host file for any Adobe blocked entries.
WIN: Windows 7 - Edit the Hosts File
MAC: How to Edit the Hosts File on Mac OS X • TekRevue
In case there are any blocked Adobe entries, please delete them and then save the file.
Regards,
Sheena