Settlers® 3: Ultimate Collection - Serial Request Frustration

I have tried:

If you are using GOG GALAXY, select the game and click the customization icon (next to PLAY):
→Manage installation
→Verify/Repair

If you are not using GOG GALAXY, open the game's directory (default: C:\GOG Games\Settlers 3 Ultimate). Look for the file regs.cmd (windows command script), right-click it and select 'run as admin'.

I have the serial but it says that is invalid.

This is soo frustrating!