how to download libreoffice to macpro OS version11

Are you getting any messages? If so, what to they say?

This article may help:

Safely open apps on your Mac - Apple Support

It discusses apps that Apple have( a) not notarized or (b) considers to be from an "unidentified developer." It also shows how to install in such cases where you trust the developer (scroll down to near the bottom).

You'll have to ask LibreOffice if and why their app is not compatible:

open office & libra office - Apple Community