There may be a bug with the Google My Business Google Posts feature. There is a spike in the number of complaints about rejection notices for Google Posts. So when a business owner posts an update to Google Posts, those updates are more likely to be rejected now.
Joy Hawkins said on Twitter "There are a lot of people experiencing issues right now with rejected Google posts. Google is aware and looking into it. For now, I would simply try reposting the same thing, and if that doesn't work, just wait a few days."
Here is a screen shot from the Google My Business Help forums with what a rejection notice looks like from Google Posts:
There has been a spike in complaints about these rejections in the Google support forums. It is unclear if this is a bug or Google is becoming more strict with Google Posts? This happened before last year.
Forum discussion at Google My Business Help and Twitter.