No updates available on my MacBook Pro

Hello,

Here are my MacBook details:

MacBook Pro (Retina, 15-inch, Mid 2015)

It currently runs on macOs Sierra 10.12.6, and the problem is that I cannot update it to a later version. When I click software update and try to pull updated, I receive the message "No Updates Available".

Please note, I tried to remove app store cache and restart the system, but without much success.

Additionally, I have recently done factory reset and I'm not sure if this is related.

I'm looking forward to get this issue fixed. Thanks in advance for any answers.

Armine,

[Re-Titled by Moderator]