Open YouTube Studio
Sign in to your account
Click Content
Select the video you want to edit
Click Details
Scroll to Comments and ratings
Choose Allow all comments
Click Save
To turn on comments for a channel
Open YouTube Studio
Go to Settings
Click Community
Open Defaults
Set comments to Allow all comments
Click Save
To turn on comments for a new upload
Upload the video
In the Details page, find Comments and ratings
Select Allow all comments
Finish publishing the video