I made individual beef Wellington. I seared my beef tenderloin and made my duxelles and let them both completely cool ,after baking , the bottom of my puff pastry was very moist and a bit undone . Now, was it because I really only thought my duxelles was completely dry or should I bake them on a cooling rack so the heat circulates all around. My friend said I should bake them on the bottom rack of the oven

Hard to say exactly what might have caused this without a bit more detail but it could have been that you just needed to bake them a bit longer.

Often the beef is rolled in crepes first, then in the puff pastry. The crepes are used to insulate and protect the puff pastry from humidity that will ultimately be released from the beef as it cooks, which in turn could result in a soggy crust. Cheers!