Error when Subtracting 2 floats. [ISSUE]

Event: So when I have $10 in the game, I should be able to buy a "Flyer" item.

Problem: Unity shows this error when I try do replicate the event:

ArgumentException: method arguments are incompatible System.Delegate.CreateDelegate (System.Type type, System.Object firstArgument, System.Reflection.MethodInfo method, System.Boolean throwOnBindFailure, System.Boolean allowClosed) (at :0) System.Delegate.CreateDelegate (System.Type type, System.Object firstArgument, System.Reflection.MethodInfo method) (at :0) UnityEngine.Events.InvokableCall`1[T1]..ctor (System.Object target, System.Reflection.MethodInfo theFunction) (at :0)

Code:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class MoneyClick : MonoBehaviour
 {
     public float cash;
     public GameObject adbutton;
     public Text cashAmountText;
 
     public void ClickForMoney()
     {
         cash += 1f;
         print(cash);
     }
 
     public void Update()
     {
         cashAmountText.text = "$" + cash;
     }
 
     public void BuyItem(float price)
     {
         if(cash == 10)
         {
             cash -= price;
             StartCoroutine(Flyers());
             print("Flyers Bought");
         }
 
         if (cash == 25)
         {
             cash -= price;
             StartCoroutine(Posters());
         }
     }
 
     IEnumerator Flyers()
     {
         yield return new WaitForSeconds(1);
 
         cash += 0.1f;
     }
 
     IEnumerator Posters()
     {
         yield return new WaitForSeconds(1);
 
         cash += 0.5f;
     }
 }