Bought Macbook May 2020 MSOFFICE 365 & outlook installed & activated (not subscription) on macbook air Jun 2020, working great until MacOs reinstall yesterday

Microsoft 365 is a subscription service. if you don't have a subscription, you can't use it.

I you purchased a legitimate standalone copy of Microsoft Office, you received an activation key. If you can't find it, you're most likely out of luck. You will either have to subscribe to MS 365, remove 365 and purchase and install Office with a valid license key, or find an alternative office suite.

If you are not able to find your license key, or don't own one, you need to buy a compatible version of Office. Either a Microsoft 365 subscription or the standalone version.

But before I sprung for it, I'd go to all my favorite web vendors where I might have bough it and search history. For instance, Amazon keeps a full history of your digital purchases with the keys.

My experience with MS chat has been rather good actually. Twice they helped me activate Office 2016. It was painless and took very little time once I proved I owned the software. I had my keys, but, the point is MS was very good at helping.

if Microsoft Support gets you nowhere, you can re-buy the package. Microsoft can require re-buying for these cases, too. Or find an alternative office suite.