Why can't we find the radius of single atom?

Answer 1

The radius is the distance from the center of the nucleus to the outer boundary of the outermost electron, and since we can't really know the exact position of that electron (Heisenberg Uncertainty), we can't really measure the exact radius of the atom.

Answer 2

Single atoms, at this time, are impossible to isolate, hence it is impossible to find the radius of a single atom.