There are other problems with it but praying to a created being vs the Creator is a huge problem.
If the Hail Mary were worshipping Mary, that would be a huge problem; but that is not what’s going on. Per Webster’s, to “pray” in a general sense simply means to implore or entreat, i.e., to ask a favor. It does not necessarily imply worship. The Hail Mary is asking Mary to pray to God for us, per the principle that the prayer of a righteous person is “powerful and effective” (James 5:16). As I mentioned earlier in the thread, there are plenty of examples in Scripture of Paul praying (in the sense of asking) other Christians to pray for him.