This is a known/reported issue for NukkitX and has been for a while now.
Unfortunately there isn't anything that can be done about this from our end, as it is simply an issue with the server software - for whatever reason, natural mob spawning has not yet been implemented.
You can, however, install a plugin that will allow them to spawn naturally. You can download this plugin here: https://nukkitx.com/resources/mobplugin.3/
Note that this plugin will need to be installed manually by uploading the plugins .jar file from the above linked page to the servers plugins folder via the FTP, and then restart the server to load it. You shouldn't need to do anything else - it should work straight out of the box.
Please also note that this does not resolve all mob related issues - horse riding is still broken in NukkitX according to the reviews on the plugins page for example.