MacOS High Sierra 10.13.6 not showing up in the App Store.

Maybe...

Turned off my VPN client on my router.

Restarted MBP (running Catalina) in Safe Mode (Hold "shift key" on start up).

Visit the page above, step four, click link...

To find out if it's system wide or user specific, try this...

Open System Preferences>Users & Groups, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

Does it work in the new account?

Apple Support Article (in step 4)

https://support.apple.com/en-us/HT208969

Any diff with this link?

https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12

Just checking, is your Date & Time set correctly?

Try this Browser..

https://brave.com/

Ignore this reply;