Welcome to the Make community!
Looks like your developer application URN is incorrect, it doesn’t fit the example format shown below the field.
You already have a client ID and secret, showing that you have created a developer application. Follow the link in the description for further instructions on how to get the correct URN.
Hope this helps! Let me know if there are any further questions or issues.
— @samliew
P.S.: Did you know, the concepts of about 70% of questions asked on this forum are already covered in the Make Academy. Investing some effort into it will save you lots of time and frustration using Make later!