Correct method for slider error handling

I'm using a material design editable numerical value (percentage) with optional horizontal slider.

What is the best way to show a user that picking a certain value either using the slider or field will produce an error?

For example, selecting below 20% on the slider will produce an error. In the case when the user has selected above 20%, is it correct to show the fill colour on the slider up to the selected percentage? Should the area on the slider below 20% be shown as red? Or should the area only be shown as red if a percentage within the area has been selected by the user?