14.x uses uses a new prototype and use screencontext. To fix the code you must use
function validate(ScreenContext, errorMessage
Please see the information provided below the "Validation JavaScript" title below regarding screenContext
<Please see attached file for image>

Delete function validate(FieldContext, attributeValue, changedValue) from the 12.6 JS code and use the following for 14.x:
function validate(ScreenContext, errorMessage)