Is your Mac capable of Mojave???
macOS Mojave 10.14.6 - Technical Specifications
General Requirements
OS X 10.8 or later
2GB of memory
12.5GB of available storage (OS X El Capitan 10.11.5 or later)*
Some features require an Apple ID; terms apply.
Some features require a compatible Internet service provider; fees may apply.
Mac Hardware Requirements
For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Mojave:
MacBook (Early 2015 or newer)
MacBook Air (Mid 2012 or newer)
MacBook Pro (Mid 2012 or newer)
Mac mini (Late 2012 or newer)
iMac (Late 2012 or newer)
iMac Pro (2017)
Mac Pro (Late 2013; Mid 2010 and Mid 2012 models with recommended Metal-capable graphics cards)
Don't use FireFox for these links…
https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12
Possible workaround...
Look into the Free LibreOffice, it handles MS Office files like Word & Excel, & Page Layout…
https://www.libreoffice.org/
And thanks to VikingOSX…
Let's not overlook SoftMaker's Office 2021 suite which has familiar Word and Excel UI design, and opens these documents in their native formats.