Is this checking for errors properly? I have reproduced the output in a different environment

So in your code, when you call the format string, you do not need to enter 'number' between the curly braces. Additionally, you are missing parentheses around the contents of the print function. The .format(number) takes care of filling the braces for you, along with parentheses:

import random

start = 5

def even_odd(num):
    # If % 2 is 0, the number is even.
    # Since 0 is falsey, we have to invert it with not.
    return not num % 2

while start != 0:
    number = random.randint(1,99)

    if even_odd(number) == True:
        print "{} is even".format(number)
    else:
        print "{} is odd".format(number)

    start -= 1

While this isn't strictly applicable to this challenge (the code will still pass), a piece of advice for checking if a value is falsey, instead of checking if it isn't zero, simply check if it exists at all within python's definitions of truthy and falsey:

import random

start = 5

def even_odd(num):
    # If % 2 is 0, the number is even.
    # Since 0 is falsey, we have to invert it with not.
    return not num % 2

while start:
    number = random.randint(1,99)

    if even_odd(number) == True:
        print "{} is even".format(number)
    else:
        print "{} is odd".format(number)

    start -= 1

Because 0 is a falsey value, the loop will stop once start == 0 as that will make start cease to be a truthy.

In terms of TreeHouse error messages, the 'task # is no longer passing' error message is usually an indicator of a syntax error rather than any effect which may be acting on the prior tasks which took me a bit to realize.

Hope that helps!