Implement a Stack using a Python list (built-in methods are not allowed)

class Stack:
    def __init__(self):
        self.stack = []

    def push(self, item):
        # Add the item to the top of the stack
        self.stack.append(item)

    def pop(self):
        # Remove and return the topmost item from the stack
        if not self.isEmpty():
            return self.stack.pop(len(self.stack) - 1)
        else:
            print("Error: Stack is empty.")
            return None

    def peek(self):
        # Return the topmost item from the stack without removing it
        if not self.isEmpty():
            return self.stack[len(self.stack) - 1]
        else:
            print("Error: Stack is empty.")
            return None

    def isEmpty(self):
        # Check if the stack is empty
        return len(self.stack) == 0

# Example usage:
stack = Stack()
stack.push(5)
stack.push(10)
stack.push(15)

print("Peek:", stack.peek())    # Output: Peek: 15
print("Pop:", stack.pop())      # Output: Pop: 15
print("Peek:", stack.peek())    # Output: Peek: 10
print("Is Empty:", stack.isEmpty())  # Output: Is Empty: False

stack.pop()
stack.pop()
stack.pop()  # Output: Error: Stack is empty.