A connection or set-up issue may be triggering this serial number error. Work through the following solutions one-by-one until you're able to sign in again.
See two Adobe logos?
If you see two Adobe logos, you have access to the activation servers. Now sign in to your Creative Cloud app again.
Don't see the Adobe logos?
Check your internet connection and try to access the Adobe activation servers again. You need a secure connection when signing in to an app. Then try to sign in to your Creative Cloud app again.
For help signing in, see how to sign in to (or activate) your app.
Confirm that your payment details, billing address, and phone number are correct. Get help with updating your billing info.
Using a corporate card? Always use the name and billing address of the cardholder, not your own. Check with the cardholder or your accounting department to verify the correct name and billing address.
If you purchased an English-only subscription, make sure that the default language settings in the Creative Cloud desktop app is English (International).