Hello @Tavneet_Singh
First, be sure to upgrade to the most up to date version of Zbrush–currently 2020.1.1. This is free for registered users, and contains new features and fixes. Older versions are not supported.
Your issue here appears to be that you have dropped your tool to the canvas as Pixols. Please read this documentation on Edit Mode, then re-draw your tool on the canvas and enter Edit Mode. Pressing Ctrl-N will clear all the Pixols from your canvas.