Hi Terry,
Serial No Prompt can occur due to many reasons:
1. Adobe Entries in Host Files// Fix: Delete the Adobe Entries and try Activating the software
2. Network Issue// Fix: Exceptions to Adobe URL and Port no 80 and 443 in Firewall and proxy.
3. User is not Invited or did not accept the Invitation// Fix: Send the invite and accept
4. Admin has to send an invitation to self and accept in order to use the CC// Fix: Send the invite and accept
Please let me know if it helps.
Abhijit