Comment 2 for bug 1765341

The issue seems to be that your subnet does not contain the gateway address, and that pointtopoint configurations aren't supported by the default server installer (as well as some of its dependencies, i would think).

The alternative server installer should work, but you may need to use it to install without networking, then (after booting into the installed system) apply a netplan configuration as discussed in bug 1767607 , comment #3