I presently have 10.13.6 OS. Would like to upgrade to 10.14 to be able to do my taxes with Turbo Tax. My computer says I am up to date and don't need an upgrade.

You are getting upgrade and update confused.

In order to upgrade to 10.14, you need a minimum of:

• MacBook (Early 2015 or newer) [model identifier 8,1 or later]
• MacBook Air (Mid 2012 or newer) [model identifier 5,1 or later]
• MacBook Pro (Mid 2012 or newer) [model identifier 9,1 or later]
• Mac mini (Late 2012 or newer) [model identifier 6,1 or later]
• iMac (Late 2012 or newer) [model identifier 12,1 or later]
• Mac Pro (Late 2013; Mid 2010 and Mid 2012 models with recommended Metal-capable graphics cards) [model identifier 5,1 or later]

Model identifiers are in your system profiler report.

Unfortunately if you don't have any of those models, you won't be able to upgrade to Mojave.

Express1040 offers web based tax processing that works on High Sierra without issues.

The App Store Updates page only informs you of updates available for the OS you are running.

Installing a newer OS is called an upgrade.

Which OS you can upgrade to will be governed by which model and year your mac is.

Click on the  top left of your screen click on About This Mac to find out the year of your mac.

For older OS’s like mcOS Mojave or macOS Catalina,

Open Safari and click on this link, 

How to get old versions of macOS – Apple Support

Go to Check Compatibility to see if your mac qualifies for the OS upgrade you want.

Then go to the Download OS section and click on the OS you want to install.

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS ******.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

If you want to or are able to install Big Sur, open Safari and click on the link below.

How to upgrade to macOS Big Sur - Apple Support

If you are considering upgrading to Catalina or Big Sur be warned they do not support 32bit apps.

Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,

some updates may be free, some may be paid. Use this app to discover which apps you have

are 32bit, https://www.stclairsoft.com/Go64/ then contact the developers of the software to see

if they are offering 64bit updates.