When you have initially installed capella with an invalid serial number, merely uninstalling the software is not enough to enable you to license again. In such a case a small adjustment in the Windows registry is necessary.
Start the Windows registry editor and select the folder HKEY_LOCAL_MACHINE/SOFTWARE/capella-software/de/[Version]. There you delete the key SerNum. When you then start the program you will be asked to enter the valid serial number.