It did compile at some point, but there were issues with my server and many crashes and now I unloaded all plugins and reloading one by one.
Now I get this error with this plugin:

Error while compiling: VehicleLicence.cs(961,65): error CS1540: Cannot access protected member `BaseMountable._mounted' via a qualifier of type `BaseMountable'. The qualifier must be of type `Oxide.Plugins.VehicleLicence' or derived from it

It was a problem pertaining to my server and not this plugin. It's working now.

I just updated and am getting the same error

(20:42:08) | Error while compiling: VehicleLicence.cs(236,63): error CS1540: Cannot access protected member `MotorRowboat.fuelSystem' via a qualifier of type `MotorRowboat'. The qualifier must be of type `Oxide.Plugins.VehicleLicence' or derived from it.

Reverted back to 1.5.5. and it works fine.

leopard

I just updated and am getting the same error

(20:42:08) | Error while compiling: VehicleLicence.cs(236,63): error CS1540: Cannot access protected member `MotorRowboat.fuelSystem' via a qualifier of type `MotorRowboat'. The qualifier must be of type `Oxide.Plugins.VehicleLicence' or derived from it.

Reverted back to 1.5.5. and it works fine.

Make sure you are updated with Oxide.