A MacBook Pro from 2015 can upgrade to macOS High Sierra, macOS Mojave, macOS Catalina and macOS Big Sur.
The thing you have done wrong is you have downloaded an update for macOS High Sierra,
these updates can only be installed if you are already running an older version of macOS High Sierra.
Moving from one OS like El Capitan to a new OS like High Sierra is called an upgrade and requires you
to download the full installer app from the App Store.
If you decide you want to upgrade to one of the newer OS's I would recommend that you still upgrade to
macOS High Sierra first s this will better prepare your mac for the new APFS file system.
To get the full installer app you must open your Safari browser, others will not work fo this.
Click on this link,
How to get old versions of macOS – Apple Support (UK)
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 free 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.