Hi @saraw7349,
The redirect_uri needs to match the setting in your API application as well as in your integration. I would encourage you to check our API documentation to ensure your oAuth flow is set correctly.
You did not mention which version of the API you are using so here are the current URLs to both versions.
v3: https://v3.developer.constantcontact.com/
v2: https://developer.constantcontact.com/
... View more
