If you try to set a tolerance of -0.000 or +0.000, your entry will be reset to ±0.005. Claris CAD considers a +/- 0.000 tolerance an error, because in such situations it would be more appropriate simply to set the tolerance to 0 (without a sign).
You must enter in a zero, without a plus or minus sign, for Claris CAD to set the tolerance to zero.