Microsoft office for OS X El Capitan 11.10.6

Your machine is not capable of running anything higher than macOS High Sierra (10.13) assuming it is a late 2009.

If it's a mid 2009, it can't run anything higher than OS X 10.11.6 (El Capitan)

You can not purchase a version of MS Office that will run on that machine at all anymore.

You might look into Libre-Office. It's open source and can handle most MS Office documents.

I sincerely hope you did not spend much money on that machine. It is thoroughly obsolete.

Microsoft Office for Mac 2008 should be OK on El Capitan.

Keep your eye on eBay .

Microsoft no longer sells or supports any version of office that is compatible with that El Capitan.

Office 2019 (or a Microsoft 365 subscription) requires macOS Mojave.

Also check Open Office from which Libre Office evolved. I have an early 2008 iMac running El Capitan, a perfect combo, it still accomplishes everything I require quickly + reliably, pretty problem free, not thoroughly obsolete at all.