After the work item is reviewed, a team member approves the work item to be ready to promote. Work items aren’t available for promotion through the pipeline until they are in the Ready to Promote status.
You can click the toggle again to indicate that the work item isn’t yet ready to promote. This toggle is available until the work item has been promoted.
If you merged the pull request directly in the source control repository, the work item is automatically moved into the Ready to Promote status and can’t be changed back. Because the change is now in the next stage's branch, your only option is to complete the promotion.
For guidelines regarding how many work items to have in a pipeline stage and how many to promote at one time, see Promote Work Items Through Your Pipeline.