Regarding your issue, I think this is the same as reported here:
So since we don’t have a macOS 10.14 available, unfortunately we’re not able anymore to build on that system although LibrePCB should still run perfectly fine when building it from sources.
I just realized we still specify macOS 10.14 as minimum requirement on the download page, I’ll correct that to macOS 11.
Btw, in the linked issue I asked what reasons exist to continue using the outdated macOS 10.x but I never got a response. Maybe you could enlighten me? It might help me (as a non-macos user) to better understand the requirements of the macOS user base.