this is the other problem I have with my Mac Book Pro too. I cannot download anything now. I need help with this one also
MacBook Pro 13″, OS X 10.11
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You need to update your MacOS to v10.13 High Sierra.
https://support.apple.com/en-us/HT208969
That means you need a MacBook Pro from mid-2010 or later. If your Mac is older, you cannot buy/run the current version of Microsoft 365. In that case, you might consider OpenOffice, Google Docs, or Apple's own suite (Pages/Numbers/Keynote).