Public method doesn't show up as a selectable option in a button's On Click event

A method will only show up for selection in the event's menu if it's public and one of the following is true:

• It takes no parameters (eg. public void DoThing() {...)

• It takes only one parameter, of a primitive type (like bool, int, float, string...)

• It takes only one parameter, of a type that derives from UnityEngine.Object (like GameObject, Component, ScriptableObject, or asset types like Material...)

A method with 2+ parameters will never show up in this menu. Neither will one with a parameter that's a custom serializable class, struct, or enum, unfortunately.

So here's how I might set this up instead (taking some guesses at what you're using it for)...

public class ButtonManager : MonoBehaviour {

    // Use this structure to store everything you want
    // to modify for a given button click event.
    [System.Serializable]
    public struct ButtonAction {
        public string name;
        public TextMeshProUGUI textToShow;
        public Image imageToShow;
        public Button buyButton;
        public Button activateButton;
    }

    // Array of the structs above, to store
    // our library of button actions.
    [SerializeField]
    ButtonAction[] _actions;

    // Our button click handler now only needs a single parameter.
    public void DoAction(string actionName) {
        // Find matching action in our array, and report error if we can't find it.
        int index = System.Array.FindIndex(
            _actions, 
            (a) => System.String.Equals(a.name, actionName, System.StringComparison.OrdinalIgnoreCase)
        );
        if (index < 0) {
            Debug.LogError($"Could not find button action named {actionName}.");
            return;
        }

        // If we get here, we found a matching action in our library.
        // Do the necessary actions on its various members.
        var action = _actions[index];
        action.textToShow.gameObject.SetActive(true);        
        action.imageToShow.gameObject.SetActive(true);        
        action.buyButton.gameObject.SetActive(true);        
        action.activateButton.gameObject.SetActive(true);
    }
}

This gives you an expandable list in the inspector that you can use to associate your text/image/button references to a specific "action", keyed by a name (helpfully displayed in the fold-out's collapsed state):

And because our DoAction(string actionName) takes only one parameter and it's a primitive type, we can select it from a UnityEvent in the inspector:

You could look up the button action by its index instead of by name, but I think using the name here is more legible to the designer setting this up, so it's easier to understand how it should work, spot bugs, etc.

I'm usually not a fan of "stringly typed" code, but here I'd argue it's no more error prone than needing to provide 1-4 magic number indices to a function. Searching through the list doing string comparisons is not the fastest, but this is code that will run at most once per frame, only when the player clicks, and iterates a list that's probably no more than a few dozen entries long, so this is not the place to micro-optimize. Especially not at the cost of clarity for the developers wiring up these menus.

I'd also question whether you need separate text/image/button objects for each item these buttons can turn on/off, or if it would suffice to have one "template" object set up that you populate with new strings/sprites/etc. when a new item is selected.