If Download Surgeon has been previously licensed on your Mac and you then update to Catalina
it is breaking the license on your machine and you will likely see an " Incorrect Computer ID"  error.

Please do the following which should fix this issue:

Open DS3 in DEMO mode.
Go to Help >> Deactivate Machine. >> Confirm deactivation.
Close Program
Open again, this time choose "I am a customer"
Now enter your registration data and it should generated a new license and you should be set to go.

If this does not fix the problem, please complete a Help Desk Support ticket.