By generated access_token, do you mean you created and installed an app onto a test merchant, then went through the OAuth process to create an access_token? Did you make sure you had the proper permissions set on the app? Does the merchant at which you're making the call have the app installed?
Yes, sounds like you have everything you need. Two things I would check are, make sure permissions for the app is set correctly in the App's settings. Second is, once the permission is setup correctly, reinstall the app on the test merchant. The new permissions will need to come down to the api token by reinstalling.
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