>printf("Please insert integer:\n");
You ask for an int, but scanf() for a float. Make up your mind which one!
at this point, opr has not been assigned a value, so you cannot test it and guarantee it the correct response.
When you layout your code correctly, you can see a logic problem here:
counter is always greater than 0, therefore you won't be scanf()'ing for opr.
if(counter > 0)
ans += num;
ans -= num;
ans /= num;
else scanf("%[+-*/=]", &opr);
There maybe some more, but it'd be best if you sort the logic out first....