I didn find this option to "Reply privately to author". I have only "Reply to the author", but says I dont have permission.
Anyway... The error is occuring directly on your Golang library, not mine.
The error is: Error 403: Multi-client accounts cannot access this method. Please use a sub-account instead.\n\n"}
We are using the same service-account we use on PHP library (your library) implementation.
Do you have some e-mail where I can send our service-account json and prints of our implementation using your library?
Thanks in advance.