You get an error similar to response_type is missing

This is an easy one to diagnose, and to fix. When you make an authorization request, that request must include several mandatory parameters (see the article Authorization code + PKCE grant type for more information). As it turns out, response_type is a required parameter; if you forget to include response_type you’ll get an error message like this:

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Likewise, scope is a required parameter. Leave out the scope parameter and you’ll get this error:

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To avoid these problems, just check your authorization request and make sure it includes all the required parameters.