You have to use else id condition in this type of program...
score = int(input('Enter score:'))
if score in range(1,3):
print('Modifier is: -4')
else if score in range(3,5):
print('Modifier is: -3')
else if score in range(5,7):
print('Modifier is: -2')
else if score in range(7,9):
print('Modifier is: -1')
else if score in range(9,11):
print('Modifier is: 0')
else if score in range(11,13):
print('Modifier is: +1')
else if score in range(13,15):
print('Modifier is: +2')
else if score in range(15,17):
print('Modifier is: +3')
else if score in range(17,19):
print('Modifier is: +4')
else if score in range(19,21):
print('Modifier is: +5')
else if score in range(21,23):
print('Modifier is: +6')