Let's start with System Preferences>Software Update, that feature did not exist until macOS Mojave 10.14 was launched.
The App Store Updates page only informs you of updates available for the OS you are currently running.
Any other updates that you may have downloaded like 10.13 Supplemental and Combo 10.15.7 will only install
if you are running an earlier version of the OS they are designed for. For example the 10.13 Supplemental would only instal on a mac running macOS High Sierra 10.13, and the Combo 10.15.7 will only install on a mac running macOS Catalina 10.15.
Moving from one OS to a newer OS is called an upgrade and can only be done by downloading the full installer application
which can be downloaded via links to the App Store.
Your mac is a 2015 model which means it can upgrade to the latest macOS Big Sur 11.2.1.
Open your Safari browser and click on this link,
How to upgrade to macOS Big Sur - Apple Support
Please read everything in Section 1particularly about the amount of storage space required
before installation, and take into account the installer app is 12.24GBs in size itself.
The installer app gets downloaded to your Applications folder as Install macOS Big Sur.app.
Although it is possible to upgrade direct from macOS Sierra to macOS Big Sur I would recommend that you upgrade
to macOS High Sierra first. The newer OS's from High Sierra to Big Sur run on the APFS file system and upgrading
to macOS High Sierra will prepare your mac for this.
To upgrade to macOS High Sierra open your Safari browser and click on the link below.
How to get old versions of macOS – Apple Support
Go to the Download OS section and click on macOS High Sierra.
This will redirect you to the App Store High Sierra download page
it should say Get or Download.
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 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.
