Error message "Invalid radius" is returned when attempting to input a curve radius using the command Geometry > Horizontal Curve Set > Define Curve.
A common reason for this message is due to the Geometry Settings in File > Project Options > Geometry Tab. If the Curve Definition is set to "Chord" and Measure is set to "Along Chord", you need to pay close attention to the value for Degree of Curve Length. By definition, when creating a curve using chord definition, the radius must be greater than half the value for the Degree of Curve Length. For example, if the Degree of Curve Length is set to 100, the radius must be greater than 50. Anything less will return the error message "Invalid Radius".