HTTP Request Fails to Update Permission Group

Hello All!

I'm trying to update a SharePoint permission group with Users as they are added to a SharePoint list. I'm using the HTTP Request to update the permission group, but it keeps failing with a BadGateway message or it times-out. Is this even possible to do with a Flow or, am I doing something wrong? Screenshots of the Flow and Error Message are below.

Thank you in advance for any help!
Patti