I've been going through the setup for quite some time due to lack of documentation.
Plugin lists support for 1.8.8 - 1.13.2, it actually doesn't support 1.13.1 (Link. This was not something that jumps out at a person troubleshooting.
The instructions are not correct for installing as the secondary world generator (multiverse) as ""forceBukkitGeneratorAPI": true" may have been removed in previous versions?
There is no documentation regarding included configurations or how to use them.