First, lose that call to int - you're converting a string of characters to an integer, which isn't what you want (you want to treat each character as its own number). Change:
inp = int(input("Enter a number:"))
to:
inp = input("Enter a number:")
Now that inp is a string of digits, you can loop over it, digit by digit.
Next, assign some initial value to n -- as you code stands right now, you'll get a NameError since you never initialize it. Presumably you want n = 0 before the for loop.
Next, consider the difference between a character and an integer again. You now have:
n = n + i;
which, besides the unnecessary semicolon (Python is an indentation-based syntax), is trying to sum the character i to the integer n -- that won't work! So, this becomes
n = n + int(i)
to turn character '7' into integer 7, and so forth.