I managed to purchase a one time subscription with license key however it does not allow me to continue and prompting a newer version of 10.14 or higher. I bought a second hand Mac (2011)just for research purposes and was looking to install office 2019 in it. For the best user experience and longest support lifecycle, we recommend upgrading to macOS Big Sur 11.0 or above. installing office 2019 on Mac OS high Sierra 10.13.6. We recommend you upgrade to macOS Mojave 10.14 or above. To ensure a smooth transition we recommend that you upgrade machines accordingly. Bugs and issues found on or specific to unsupported operating systems will no longer be investigated or fixed.New installations on unsupported operating systems will be blocked.It is not advised to continue to run on these unsupported operating systems. These clients will continue to run, but their functionality will no longer be tested. Existing OneDrive sync client installations on unsupported operating systems will stop updating.

How users will be impacted after this change has rolled out:

Action: review and update impacted machines as appropriate.Note: Both macOS Sierra 10.12 and macOS High Sierra 10.13 have been out of Apple’s mainline support since September 2019 and November 2020, respectively.

When a new major version of macOS is released, we will support the new version and the previous two versions. Going forward, only the three most recent versions of macOS will be supported. As such, we are ending support for the OneDrive Desktop application (sync client) on macOS Sierra 10.12 and macOS High Sierra 10.13 and focusing our resources on new technologies and supported operating systems. The OneDrive team is committed to bringing you the best, most secure experience possible across your devices. If your organization is not using macOS you can safely disregard this message.

Office 365 mac os sierra