General::tol
The value of the Tolerance option cannot be None:
NullSpace[{{1.2, 3.4, 5.6}, {1.2, 3.4, 5.7}, {1.2, 3.4, 5.8}}, Tolerance -> None]This shows a valid value for the Tolerance option:
NullSpace[{{1.2, 3.4, 5.6}, {1.2, 3.4, 5.7}, {1.2, 3.4, 5.8}}, Tolerance -> 0.1]