Thank you for reaching out to Constant Contact API Developer Support and for your patience. My team is here to assist outside software developers with questions about building into Constant Contact's API.
It looks like you are using the V3 API documentation and endpoints with a V2 API key and access token, which is likely the reason you are receiving an error. The app that you have registered in the Dev Portal should have your V3 API key, and the API endpoint tester at v3.developer.constantcontact.com also accesses the V3 API key that was created. To resolve this, you’d want to use the V3 API OAuth2.0 Server Flow to authenticate the connection and generate an access token and refresh token. Those instructions below:
V3 OAuth2.0 Server Flow https://v3.developer.constantcontact.com/api_guide/server_flow.html
For your privacy and security, we’ve removed the image of your V2 API Key and Client Secret that was included with your post, and have also revoked the token (the Client Secret was no longer secret). If you do need to use the V2 API, I’d recommend generating a new key. The V2 API keys generated through Mashery have a different authentication flow and endpoints than the V3 API.
V2 API Documentation https://v2.developer.constantcontact.com/docs/developer-guides/overview-of-api-endpoints.html
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