Facebook has limits on the number of ads, ad sets and campaigns per ad account as described here. If this error happened it means you need to delete some ads to comply with Facebook limits and make sure Facebook allows creation of new ads.

As an option, you can use "Delete" task without automated rules to clear your accounts from old ads. Delete action is permanently removing ads when conditions performed. You would not be able to restore the item once it is removed by the rule. So if you decide to use automated rules for clearing old ads, we recommend double checking that all filters are targeted only at ads you want to delete.

The rule below, as an example, would be deleting the ads with inactive, completed or rejected delivery if they were created more than 9000 hours ago. The more filters you add to narrow down the scope, the better.