Open YouTube Studio
Sign in to your account
Click Content
Select the video
Click Details
Scroll to Comments and ratings
Choose Allow all comments
Click Save
To turn on comments for a new upload
Upload the video
In Details, go to Show more
Find Comments and ratings
Select Allow all comments
Save the video
To turn on comments for a channel
Open YouTube Studio
Click Settings
Click Community
Check the default comment setting
Set it to Allow all comments
Save changes