FiveSeveN-

It's not missing it's just that first checkbox. I guess it can be reworded slightly but the V+ dev decided that one section would have a slightly different wording compared to the enable=true for each of the other sections simply enabling those sections. You need to have that first checkbox enabled to enable the second one. I don't quite understand why it is a section enabler boolean.

You have Enforce Mod Version Check unchecked.

Basically if you have this unchecked it is removing the @0.9 from the version string which in turn allows non V+ clients to connect. The caveat to this is that your client side valheim_plus.cfg will need to have the enforcemod=false as well (it does not auto sync) which would have to be set to any V+ connecting clients to that server.
So the solution is to either keep it checked and only allow V+ in or manually update every V+ client to have enforcemod=false on their valheim_plus.cfg client-side.
Added a little information to our config tool so that future customers are aware of this limitation.

I, like many people kept getting the "Incompatible Version" message when trying to connect to our server after installing valheim+ 0.9 . What solved it for me was after installing Valheim+, going into the Modify Valheim+ Config section in the control panel, and just scrolling down to the bottom to hit save configuration, even if nothing is enabled/changed. I hadn't done this prior as I was just trying to connect to see if it was working before getting into the config, but it might be that there isn't a default config file server side until it's generated from saving the config for the first time in the control panel.

That actually comes from nexus mod Valheim Plus page.
https://www.nexusmods.com/valheim/mods/4
 

This actually worked, holy smokes. You have found the solution.