UPS Rest API Headache

I'm driving myself crazy trying to setup a UPS REST API in Retool. I'm new to the API game so bear with me but I'm not sure where there is a disconnect. When I go through the connection process with the URL info attached, I get to the UPS login screen and then it brings me to this error.....

{"error":true,"message":"Internal server error"}

What am I missing here? Has anyone setup a UPS API within Retool? I'm hoping it's just my ignorance in API-land.

Hey @cpetersonx can you share how you have the Oauth app set up on UPS? Thanks!