Error 403 when trying to use authorisation code flow

[joegr1]

I have built an app which uses spotify's Oauth 2.0 to log in. However it only works with users i have added to user management in the apps dashboard on Spotifys for Developers website. So the auth flow code works but i have t manually add new users.

the error 403 is confusing me. 

can anybody with knowledge on this jump on a call and help me ?