The same error happens with server 1.15.2-31.2.0, 1.15.2-31.2.36 and 1.16.2-33.0.22. I'm running a FreeBSD 12.1-STABLE server.
Run forge-<version>-installer.jar after a clean minecraft server install. The forge install runs uneventfully.
When I try to run it, the following error message is displayed
The server installer log for 1.16.2-33.0.22: Installer log(removed, see my reply)
The server installer log for 1.16.2-33.0.22: Installer log(removed, see my reply)