Apologies I'm brand new to Xwiki, I'm probably filing this in the wrong place but was requested to submit a bug on the forum so this is my best guess.
With Java 9 on macOS running the latest standalone version of Xwiki with the standard flavour preinstalled I get this error:
I fixed that by changing this line:
If it's helpful here is the output from that Java version command:
The forum post where I initially asked for help is here: https://forum.xwiki.org/t/trouble-installing-xwiki/713