If the input contains non-alphabetic characters, prompt the user to "Please enter a sentence with letters only." Eg 1: Input: Hello World Output: olleH dlroW Eg 2: Input: Hello 550! Output: Please enter a sentence with letters only

You can achieve this by writing a simple program that checks the input for non-alphabetic characters. If the input contains any characters that are not letters, the program will prompt the user to enter a valid sentence. If the input is valid, it will reverse the letters in each word and print the result.

Here's a sample code implementation in Python:

```python
def reverse_words(sentence):
    words = sentence.split()
    reversed_words = [word[::-1] for word in words]  # Reverse each word
    return ' '.join(reversed_words)

def main():
    user_input = input("Please enter a sentence: ")
    
    if any(not char.isalpha() and not char.isspace() for char in user_input):
        print("Please enter a sentence with letters only.")
    else:
        result = reverse_words(user_input)
        print(result)

if __name__ == "__main__":
    main()
```

### How it works:
1. The program prompts the user for input.
2. It checks if the input contains any non-alphabetic characters (excluding spaces).
3. If non-alphabetic characters are found, it prompts the user to try again.
4. If the input is valid, it reverses each word in the sentence and joins them back into a single string, which is then printed.

### Example Outputs:
- **Input:** Hello World  
  **Output:** olleH dlroW  

- **Input:** Hello 550!  
  **Output:** Please enter a sentence with letters only.