Hi there,
Thank you for reaching out via the forum.
Your details look fine there on the surface, so we would want to look at the headers you are sending to the API if possible please.
Typically we see this issue with the Redirect URL not matching between the MY.MYOB Developer Dashboard and the call to the API, explained here: Solving Invalid request – Support for the MYOB family of SME product APIs
We also expect to use the Oauth2.0 method, and have found some users utilising the "Basic" Auth method which we are finding is no longer suported (it was previously but have noticed a change recently).
If you are using this method, please change to Oauth2.0 and this should help.
Otherwise if you are still getting invalid request, please submit a support ticket here.
Kind regards
