Asking name to be taken off a paper
My suggestion, actually, is that you don't let the perfect be the enemy of the good. You aren't satisfied with what you did. Fine. But something was learned because of it. Better. 

I'd let it go. If the PI thinks you are a co-author then defer to their experience and judgement. You haven't set science back because you tackled a difficult problem and weren't able to reach a perfect conclusion. That is, in fact, the nature of scientific research. 

But if you really don't want to be co-author, just say that. It should be fine. However, if you also don't want to be listed at all, even in acknowledgements, then you leave the others with a serious dilemma. They will have to write the paper in such a way as to avoid plagiarism of your work, if you won't let it be attributed to you. That seems to me to be the worst outcome. Something is known that wasn't previously, but it becomes difficult to let the world advance upon it.