I need to upgrade my Mac to OS 10.14 but there are no updates available. What do I need to do?

early 2103 is limited to Catalina, maximum. You can obtain that (or another slightly older one) here; if you are not already running it:

How to get old versions of macOS - Apple Support

if you are running a really old version, you may need to install El Capitan as a waypoint to get the improved Mac App Store to go further later.

The article you posted is how to Update. Updates work within one major version. the get to the next version, you would need to UPGRADE (like a new purchase, on sale for $0)

How to upgrade to macOS Big Sur - Apple Support

NB>> the requirements for computer and especially for storage spaces are substantial -- as much as 45GB free in some cases.

what model MacBook Pro by year?