All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Allaah is Most Merciful and Most Kind to His slaves and He does not burden a soul beyond its capacity. It is the duty of every Muslim to fear Allaah as much as he can as Allaah says (what means): {So fear Allaah as much as you are able.}[Quran 64:16]
Therefore, if you are not able to pronounce those letters properly, then you are not sinful and your prayer is correct. But you should endeavor to learn and improve your pronunciation as much as you can, and whatever you are not able to pronounce properly, then you are exempted from it.
Allaah Knows best.