When you do a private request to Coinbase, the message is signed so increase the security of requests. The message takes the local time and sends inside the signed message, if the local time has a difference greater than 30 seconds with Coinbase servers, the request will be rejected. So, it's important verify that your local time is synchronized, you can do this using the synchronization time method for your OS.
You can check the Coinbase Pro server time, calling method GetTime, which will return the time of the Coinbase Pro server