Can i know why do I have to play vs a Platinum V if I am unranked?

well, platinum5/4 players are usually trashes so isnt a big deal.

Yeah, getting matched with a diamond player sucks. It's basically just wasting 20 minutes.

What rank were you last season

If you are playing normal, their Normal MMR is different than their Platinum MMR, so they probably have the same normal MMR as you do.