User18 0.1.4 is the latest version and is compatible with both Flarum beta 15 and 16.
I'm not able to reproduce using your instructions (tested on beta 16 only). A freshly enabled user can create discussions and see them fine on my test forum.
There's something odd with your output. It seems like you don't have Flarum Approval enabled. You should not have been able to enable First Post Approval without first enabling Approval, and likewise it shouldn't be possible to disable Approval while First Post Approval is enabled. If Approval is indeed disabled, then it's certainly why it doesn't work. Try enabling it.
If you are able to disable Approval while First Post Approval is enabled, then there must be a bug in Flarum because Flarum should prevent you from doing it.