I have LibreOffice 7.1 Base installed on System76 POP (an Ubuntu-derivative). It doesn’t have Report Builder. The only installation instructions I see at Report Builder are for a JRE; that is not the problem, as there is already a JRE on the system, and LibreOffice already recognizes it. There are no instructions for installing Report Builder.
I looked up the file list for package “libreoffice-report-builder” for Ubuntu, and the contents thereof; no “reportbuilder.jar” can be found anywhere on the system.
System76’s “Pop Shop” app store does not show anything for LibreOffice Report Builder.
If I open the LibreOffice Extensions Manager, the closest thing it can find is BaseReportExtension, and its own page makes it obvious that it’s something entirely different.