Google has rules in place, restricting some businesses from being able to use the GMB API for publishing.
Who is NOT Eligible to Schedule GMB Posts?
At this time, Google does not allow businesses with 10 or more locations to schedule posts to GMB. More information and their reasoning can be found in Google's developer docs.
Why the Limitation?
Rather than a limitation, Google is seeing this as an advantage for small businesses as part of their SMB initiative. According to Google, they're "working to determine the best method to offer the Posts feature to large chain retailers in the future".
What Can Restricted Businesses Do?
Although we cannot post on your behalf, you can still monitor your activity feed and track your analytics using Cloud Campaign.
How Can I Enable Posting?
If you manage social media for a business that has 10 or more locations on Google My Business, at this time, the only way to schedule posts is to split the business into multiple unique businesses on Google. Each business can have at most 9 locations.
Check back to this article for updates as Google hopes to support larger businesses in the future.