Quite a lot is wrong. You are mis-spelling variables (upper/lower case first letters) so they do not match. And all the code below the first few lines are not correctly indented. The correct version is as follows:
import random
score = 0
_question_No = 0
while _question_No != 10:
num1 = random.randint (0,10)
num2 = random.randint (0,10)
num3 = random.randint (1,3)
if num3 == 1:
print("What is", num1, "+", num2, "?")
Computer_ans = num1+num2
elif num3 == 2:
print("What is", num1, "-", num2, "?")
Computer_ans = num1-num2
else:
print("What is", num1, "*", num2, "?")
Computer_ans = num1*num2
user_ans = int(input())
if user_ans == Computer_ans:
print("Congratulations!")
score = score +1
else:
print("Incorrect!")
_question_No = _question_No + 1