Good evening,  @perminder_sandh ...

Are you following the directions outlined in this Guide?  https://community.canvaslms.com/docs/DOC-26329-how-do-i-add-assignment-types-pages-and-files-as-modu... Specifically, it sounds like you would want to look at the "Add Existing Item" section of that Guide since you've already created the assignment on your "Assignments" index page.

I hope this helps.  Please let Community members know if you have any further questions about this.  Stay safe, and be well!