Upgrade to Plesk Obsidian 18.x.xx is available - Install now button is missing from Plesk Onyx 17.8 Home screen:
Known Plesk bug with ID PPPM-11188, which is planned to be fixed in one of the future product updates.
Until the bug is fixed, use the following workaround:
Connect to the server via SSH.
Run the daily "Check for updates" task:
# plesk daily -f CheckForUpdates