On Windows, the Extensions menu is missing after upgrading Plesk from Onyx 17.5 to Onyx 17.8.
It was a bug in Plesk Installer with ID PPP-35649, which was fixed approximately on March 16th, 2018.
Plesk Installer is updated automatically on each launch, so the issue should not occur during the later upgrades.
If the server is upgraded prior to March 16th, 2018, use the following solution: connect to the server using RDP and execute the below command in CLI:
C:\> plesk bin extension -i "%plesk_dir%var\ext-catalog.zip"