Some companies and educational institutions use their own activation servers to keep track of installations of software that is distributed to employees and students. They don't keep them up forever. When they move to a newer version of the same software, they will eventually decline to activate the older versions. I know this for a fact. My wife was taking courses at Indiana State and was able to download and install Office 2010 on her computer. The activation was handled by the school A few years later, the computer died, and when she tried to activate it on a new computer, the activation failed. The IT department at the school indicated that they no longer supported installations of the "old" software for the reason stated.

You may want to do as I have done. I have a Mac, but I have added MS-Windows to it. The people at Apple guided me along the way. I did have to purchase a separate copy of the Windows OS system. I'm able to switch back and forth between both operating systems if I desire, but I mainly us Windows. In the past I had purchased Microsoft Office Professional. The new version that I now have on my computer is not quite at that level. However, Microsoft offered a free version of Office 365 which I did download but have not really used.

I'm not sure what your needs are, but be careful. Not all versions do the same thing. When I had the full version of Excel I was able to do quite a bit more than what I'm able to do with the current version. One example is that the current version that I have will not copy the template of one file that has data into a new blank file.

https://www.softwarehubs.com/product-categories/microsoft-office-office-suites/office-professional/

Office 2021 for $40. I have bought from them several times. Correction: I just notice that you said Mac, this is for PC. Office for Mac is not so cheap:

https://www.softwarehubs.com/product-categories/microsoft-office-office-suites/office-for-mac/

Softwarehubs is a Microsoft Certified Reseller, and this is completely legit (you will actually get the license from Microsoft itself, and install it into your Microsoft account). I have gotten Office from other "Grey Market" resellers in the past for ~$20, but in a year or so, it may or may not still be working.