Thank you, Narsaw. I actually have the integration installed, it just isn’t showing any devices or entities. After coming across this thread No devices or entities appearing after successful Nest Thermostat Integration I am attempting to follow the instructions here: https://developers.google.com/nest/device-access/authorize and have made my way through to the final step to make the device list call… which gives me the error above. 
I’ve found the reference code blocks on the page don’t work very well out of the box. I’ve been able to google and stackoverflow my way through the previous steps editing the curl commands until they work, but haven’t managed to figure out the right format for this curl command.