The No Action Required button means that the post is good in your opinion. However, it doesn't mean you have to press it.
A post may need improvement, it may need to be flagged, it may need you to ask the OP for more clarification, etc.
Knowing that, if you try to do any of the previous actions (suggest an edit, flag, comment, etc.), the No Action Required button will be disabled, and the I'm Done button will be enabled.
If you click on (more), you can see more details about what to do in a review.
