In order to take any P level papers, you have to attempt all your remaining F’s at the same time. So in your case, you have to take F6 and then you can decide wether you want to take P1, P2 or P3 (or two of them, or all of them) together with F6.
If you only plan to attempt two papers, then F6 must be one of them. The only way round that is to enter for F6 and P1 with P2, but then don’t turn up for the F6 paper.