I have a macOS Sierra version 10.12.6. Can I upgrade this to the 10.13 so I can install Microsoft Office

That depends on the model / year of your Mac, not the macOS version you are currently running.

Go to Apple Menu->About this Mac to get those details.

See here for requirements for installing 10.13 High Sierra: How to upgrade to macOS High Sierra - Apple Support

Or skip MS Office,  the Free LibreOffice, handles MS Office files…

https://www.libreoffice.org/