Welcome to the Make community!
It appears that the data received by your webhook is invalid JSON? You might want to verify that the data received is valid first, otherwise you should contact the external service sending the data to your webhook URL.
You can verify whether a JSON is valid by copy-pasting it into jsonformatter.org
If it it valid JSON, by default the Webhook module should have automatically parsed the request body into variables.
Hope this helps! If you are still having trouble, please provide more details.
— @samliew