Is it only the Snapshot menu item that's grayed out, or others like Migrate too?
One reason could be that a backup is still in progress (or has crashed), and the corresponding flag is still set. In such a case, running another backup may solve the issue.
André
Moderator note: Discussion moved from Workstation Pro to ESXi Discussions
There can be many reasons for that. Have a look at Snapshot limitations documentation and check if your VM falls under any of listed limitations.