There's a bunch of stuff I can't draw. Motorcycles, for example. I'm also not the greatest at drawing huge robots, or people kissing, or that impossible camera angle of a person's face from below.
But give me enough time to find a good photo-reference for it, and I can make something that doesn't look half-bad, and learn some things in the process.
As for what I won't draw, the list is mostly - explicit pornography, extreme gore/extreme violence*, most types of bodily waste (no illustrated poop jokes here XD).
*) My comic contains both violence and blood, but I have no interest in drawing super-detailed epictions of wounds or internal organs. It's very, very unsettling to me, and I usually don't read comics that contain gore either.