What does 套 mean in ...绝对没有一个规则套在这里...?
My understanding is that 套 is short for 套用 here, meaning "apply mechanically".

Basically, the author conveys there isn't such a rule/principle that could work for all the circumstances, so that we don't have to worry once applying it.

With that in mind, I gather you should be able to wording it much better than I do as an English native. 

This is my shot:
(there is) absolutely no rule/principle fixing(or being applied) here, so we don't have to worry about it any more. 

On top of that, if the sentence is written as 绝对没有一个规则被套用在这里, would it be more understandable to you? In fact, it's a passive voice sentence with some omissions.