I want to receive the return value of the function and display it on the screen, but it says none

Given the state of the code, I've just started studying PythonRather than that, I needed something like this, so I roughly found it and made it.

Based on the code, it looks like you've compiled what you've looked up here and there.

You've compiled a code that doesn't fit at all.

If you've studied coding a little bit, you've organized the code in a way that you wouldn't have done, and if you want to solve it yourself, you'd better find something that can help you, whether it's on the Internet or in a book.

If you simply want to print that number every time you get a multiple of 7, the code should consist of:

print ("I will output a multiple of 7 from 1 to 100")
def seven_cal ():
    for a in range(1,101):
        if a %7 == 0:
            print(a)

seven_cal()

>> 7
14
21
28
35
42
49
56
63
70
77
84
91
98

If you want to output a composite of the results of finding a multiple of 7 in the function, construct the code in the following way:

print ("I will output a multiple of 7 from 1 to 100")
def seven_cal ():
    result = []
    for a in range(1,101):
        if a %7 == 0:
            result.append(a)
    return result

b = seven_cal()
print(b)

>> [7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77, 84, 91, 98]