Improvements made.

sum_of_digits_of_a_number.py

@@ -0,0 +1,28 @@
+import sys
+
+def get_integer_input(prompt, attempts):
+    for i in range(attempts, 0, -1):
+        try:
+            n = int(input(prompt))
+            return n
+        except ValueError:
+            print("Enter an integer only")
+            print(f"{i-1} {'chance' if i-1 == 1 else 'chances'} left")
+    return None
+
+def sum_of_digits(n):
+    total = 0
+    while n > 0:
+        total += n % 10
+        n //= 10
+    return total
+
+chances = 3
+number = get_integer_input("Enter a number: ", chances)
+
+if number is None:
+    print("You've used all your chances.")
+    sys.exit()
+
+result = sum_of_digits(number)
+print(f"The sum of the digits of {number} is: {result}")