API to get foreign exchange rates from partner center

We are attempting to use the API call detailed from
https://learn.microsoft.com/en-us/partner/develop/get-foreign-exchange-rates
to get foreign exchange rates and have followed the prerequisites for setting up an App Registration with the correct permissions and access.

we are getting aad token, after that use that token to get new token from partner center,

we can use this token for all other partner center api's except for https://api.partner.microsoft.com/v1.0/sales/fxrates(Month='201909')/$value

where we are getting this error:
{
"statusCode": 401,
"message": "Unauthorized: Invalid Authorization header"
}

can someone please advice what can be the problem, thanks.