No the Combo package can only be used to update an existing installation of macOS Mojave.
In order to install macOS Mojave you need to download the full installer app through the App Store.
If your mac can upgrade to macOS Mojave then it can also upgrade to macOS Catalina.
For Mojave or Catalina,
Open Safari and click on this link,
How to get old versions of macOS – Apple Support
Go to the Download OS section and click on the OS you want to install.
If you choose Mojave it will download to your Applications folder as Install macOS Mojave.app.
It should be 6.05GBs in size. (Catalina does the same it is 8.26GBs in size.)
This is all you need to install a full working version of macOS Mojave 10.11.6, this is all you need.
The installer app self deletes after installing so it is advisable to make copy of it on an external
drive for safe keeping in case you ever need to install it again. Or even better make a bootable
USB flash drive installer using the instructions in this article.
How to create a bootable installer for macOS – Apple Support (UK)
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 be warned it does 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.