Why do I get "Missing" in the Editor when I select a method to call on Button?

[Wampster]

I have a “Return” Button in my MissionCompleteMenu (UI). I have attached a Missions class in the scene and it has a method called MissionComplete(string _mission). But, although it lets me attach the method to the button, it tells me that it is Missing.

Attached is a screenshot showing the code and the error.
Does anyone know why the Editor is doing this? I assume it is human error, but I don’t understand what the error is.

[Wampster]

It seems to have been because it was static.

I moved the method to MissionCompleteMenu.cs and made it public. Now the method works as intended.