On some (not all) themes it will allow you to set this through the customization tab.

To get here do the following:

Followed by (this will generally be down the bottom)

Then this.. Select whether you want to allow this for both posts or pages

Make sure you click the save button.

This will allow this SITE WIDE.. meaning every post / page you create from this point on-wards will allow comments.

If you dont have this option within your theme continue to the next step