There is a Meta limitation as to the number of ad sets that can be contained in the parent entity (for Advantage Campaign Budget aka CBO campaigns). If you see this error, it means that this limit has been reached for the campaign used for this specific post boosting rule.

In order to get the posts to be boosted again, we suggest duplicating this rule, then please make sure that the duplicate has all the settings you need, and launch the rule live. The new rule will create a new campaign, so the rule will start to boost the posts again.

Alternatively, you can delete old ad sets from the campaign either in Meta Ads Manager, or using "delete" task in Bïrch automated rules.