i can't download microsoft outlook . "can't be installed on "Macintosh HD" because macOS veersion 10.15 or later is required. What will i do . Tnx

Microsoft has retired everything but the current Microsoft 365 subscriptions, and the single-purchase Office 2019 for Mac, and Office 2021 for Mac. If you attempt to install any Office 2011 for Mac, or Office 2016 for Mac product on macOS Mojave or older, you will be blocked by Microsoft and the fact that they have disabled unsupported product license servers.

Try downloading the 2016 version. It will work on MacOS High Sierra.

Good luck👍