Is there a point on the secp256k1 curve for any given X coordinate?
The size of secp256k1's coordinate field is 2256 - 232 - 977.

That means there are only 232 + 977 (about 4 billion) possible 32-byte combinations that are not a valid coordinate.

Only slightly less than half (around 2255 - 1.17 * 2127) of those are the X coordinate of a point on the curve (in fact, for every valid X coordinate, there are either exactly 0 or exactly 2 points on the curve).