This article is for the people who see this red error icon next to a rule in post boosting:
Most likely, that's because some of the other automated rules stopped that post boosting campaign or you did it manually at some point. The post boosting rules couldn't have done that.
You can try activity history in Ads Manager for that campaign and see what stopped it.
In order to fix this issue create a new rule from scratch or simply restart the campaign and if that doesn't work hit us up in Intercom chat or at email@example.com.