If you are unable to approve a milestone document, you likely aren't a member of the appropriate approval group. 

From the project, navigate to Settings followed by Approval rules. Please note that you may need the correct project permissions to be able to see this page. Only organizations owners are able to see the approval rules page by default. 

From the Approval rules page, scroll down to the Release approvals. Since a milestone document is designated as a release document, you will need to be a member of the Release group to approve milestone documents. 

Follow up questions: I am a member of the Milestone group. Why am I unable to approve milestone documents? 

If you have Require controlled milestones enabled in the project settings, members of the Milestone group are only able to approve the Milestone itself. However, they are unable to approve Milestone documents. 

Follow up question: I am a member of the Documents group. Why am I unable to approve milestone documents?

As a member of the Documents group, you are able to approve documents that exist as part of the electronic document management system (EDMS). You are unable to approve Release documents, which milestone documents are a part of.