From the “tree->Print()” output in your original post above (for a variable size array, you should see there something like “Jet_pt[nJet]”) and then from your very last trial (the one that returned 227.625), it is clear that the “Jet_pt” is a single ordinary “Float_t” value.
If you don’t know how to deal with your tree, see how various flavours of automatically generated “analysis skeletons” deal with it.