This software requires macOS version 10.15.0 or later.

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence

macOS Catalina is compatible with these computers If the computer Is Listed - it qualifies. If it is not listed - it Does Not qualify for Catalina

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal  operation of the OS and may even Corrupt the OS requiring a Reinstallation

Microsoft follows Apple's three-year sliding window of operating system support. So, once Monterey shipped last Fall, Microsoft went from requiring macOS 10.14.* (Mojave) to macOS 10.15.* (Catalina). New installations of Microsoft 365 or Office 2019/2021 for Mac on operating systems older than Catalina will now fail. There may also be AutoUpdate failures for these Office suites that are already installed on Mojave because AutoUpdate is expecting Catalina.

You can take this up with Microsoft.