how to add nofollow tag to paid links in the widget of sidebar area
You write the simple code
<a href="https://www.yourdomain.com" rel="nofollow">Your Text link</a>
With the help of rel="nofollow" you will easily make any link to nofollow.
As you're on the CMS & Wordpress forum, could you tell us what CMS and widget you're using? Whilst @soumyadip is correct in the pure HTML sense, it is likely that you'd need to look closely at the widget code to see whether you can implement it in the way suggested.