You can only end a review after you have combined a reviewed presentation with your original presentation.
Note Although reviewed presentations can be combined with your original presentation at any time during the review process, when you choose to end a review, you will permanently end the ability to combine any more reviewed presentations with your original presentation.
Tip
Microsoft PowerPoint will automatically end the review for you if you have applied the reviewer changes you want, deleted all change markers (change marker: An icon that designates a change made by a reviewer to a presentation.), and saved your presentation.