Microsoft Office only supports the last three versions of macOS. So if you want to install it onto an older version of macOS, you need to know what the last supported version number is. You can download that from Update History for Office for Mac, or use the direct links provided here.
Big Sur (11), Catalina (10.15), and Mojave (10.14) support the current version of Office, which, as of July 2021, is 16.50.
High Sierra (10.13): The last supported version is 16.43. Direct link: Office for Mac 16.43
Sierra (10.12): The last supported version is 16.29, with Word, Excel and PowerPoint updates to 16.29.1. Direct links: Office for Mac 16.29 / Word 16.29.1 update / Excel 16.29.1 update / PowerPoint 16.29.1 update
El Capitan (10.11) and Yosemite (10.10): The last supported version is 16.16.27 (aka Office 2016). Direct link: Office 16.16.27
Mavericks (10.9), Mountain Lion (10.8), Lion (10.7), Snow Leopard (10.6), Leopard (10.5): The last supported version is 14.7.7 (aka Office 2011). Direct link: Office 14.7.7
Tiger (10.4): The last supported version is 12.3.6 (Office 2008), with optional Entourage Web Services Edition 13.1.6.
Panther (10.3), Jaguar (10.2): The last supported version is 11.6.6 (Office 2004).
Puma (10.1): The last supported version is 10.1.9 (Office v.X).
Cheetah (10.0): No supported version of Office. Cheetah was, for all intents and purposes, unusable.
Mac OS 9: The last supported version is 9.0.6 (Office 2001), and for Exchange users, the unrelated Outlook 8.2.2 (Outlook 2001).