Invalid client error while calling refresh token api

It probably means your appKey or appSecret is wrong.

In your code there is "MyaccessToken", how did you generate it?

Could you please provide the following:

1. Are you using the RingCentral JS SDK? If yes, which version?
2. Description of the 'endpointId' variable's assigned value
3. The complete error response with headers/body from this request

This will help in figuring out what is happening here.

You will need to view the documentation, because Tyler is correct, you are using the incorrect Authorization Header key:value. https://developers.ringcentral.com/api-docs/latest/index.html#!#RefRefreshTokenFlow

What this means is in your example you set the "Authorization: Basic {{myAccessToken}}", but this is incorrect because you SHOULD NOT be using your "access_token" as this value, you should instead be using the result of Base64 encoding your {{appKey}}:{{appSecret}} (which is the same value you used to obtain the 'access_token' initially).

Also, please pay special attention to the description for endpointId...

If not specified, the previously specified or auto-generated value is used by default

Yes you don't need to put access_token in Authorization header. All you need to put is Base64 format of clientId and client secret as follow:

Authorization Basic <clientId:clientSecret in Base64>

Access_token are need to pass to the actual main APIs as type Bearer to get the response