Unless your surgeon says otherwise, there is no reason you can't kneel once you are sufficiently healed.
But, as EalingGran said, many folks don't like the sensation of kneeling on implants. It takes time to adapt to the sensation, but you can work towards getting comfortable with it.
After my first TKR, I practiced kneeling on a thick chair cushion, which offered good padding for my knee. It took until I was about 4-5 months post-op before I could do that for any length of time. As my new knee became accustomed to that, I switched to a thinner cushion, then to a folded towel and finally to my yoga mat. I still don't like kneeling on the hard floor, though!