Why can't I update my MacOS for High Sierra. Office 365 updates cannot be completed without updating the OS. MY subscription is not due until November.

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.

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.

Make a Time Machine Backup Before attempting. Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina & Big Sur supports only 64 bit and there is no workaround.

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

To add to P. Phillips notes. The age of the OS also governs what version of Office 365 you can install. Your version of Office is 64 bit, but that's not the problem.

Microsoft's current rule is they support the three most current versions of macOS for Office 365 and 2019. This is relevant to a user who has not yet installed either and activated the suite.

You are grandfathered in with your current version of 365. Since it's already activated and running, you can continue to use it despite the three OS rule. But, you also cannot update it beyond its current version (and it's pretty far behind now) because those updates would be looking for Mojave, Catalina or Big Sur and won't function in High Sierra.