Hi Naveen, you could use the ‘length($object/attribute) > 20 ’ in a split action in a microflow to determine if you need to show the validation message or not.
Alternatively you can use the default settings on the textarea to predetermine the lengt of the input and feedback to the user. This way you possibly don't need the microflow and the users gets the feedback directly. Also see https://docs.mendix.com/refguide/text-area