The error "token not authorised" might indicate that you're passing just the API key for tokenised authentication and not an encoded username+API key combination.
To successfully authenticate to the Assembly Payments API, there are two methods to use:
Basic authentication using your username and API key.
Tokenised authentication using a Base64 encoded username and API key to pass a single authentication token.
For more information on API keys and authenticating with Assembly Payments, refer to the guide on API Keys.