Couldn't map to url , {"status":401,"error":"Unauthorized","message":"No message available","timeStamp":"Sun Feb 03 11:10:13 IST 2019","trace":null}

I’m trying to develop an add-on which gives Estimated due dates of all users in a sprint .
so I started creating login page for admin( I know basic auth is now deprecated but what to do, oauth seems to be difficult) .
So i’m redirecting login page to a url , but unfortunately I get this error !
Any help?