This works on my atom editor, why won't it work on the workspace?

The challenge wants you to split on all whitespace, your code splits on spaces which is a type of whitespace, but there are lots of other characters considered whitespaces. Like line breaks, tabs, and other things like that. The code works on the example input since that only uses spaces, but the example input the challenges shows you is often different from the value they actually pass to the function. They often differ in order to make it harder to cheat the challenge checker by hardcoding your return to the right values.

Also while list comprehensions can be fun to write they are not always the right tool for the job, to be honest your code is extremely complex compared to the task it is trying to solve. For one implementing a custom string-to-list splitting method instead of using the build-in split method.

If you use the split method then by default (with no arguments passed) it will split the string on all whitespace. So if you replace your string_into_list function with split like this:

def num_list(word_list: list, word):
    num_time_words = 0
    for i in [i for i, x in enumerate(word_list) if x == word]:
        num_time_words +=1
    return num_time_words

def word_count(string_argument):
    main_dict = {}
    word_list = string_argument.lower().split()
    main_string_copy = word_list[:]
    y=0
    x=0
    for words in word_list:
        if num_list(word_list, word_list[y]) > 1:
            word_list.remove(words)
        y+=1
    for words in word_list:
        main_dict.update({words: num_list(main_string_copy, words)})
        x+=1
    return main_dict

Then your code will work, though it will still be quite heavy for the task. A simple way of solving this task is to check if the word currently exists in the dictionary. If it does update the dictionary with its current value + 1, if it does not then set it to 1.

Like this for example:

def word_count(string_argument):
    main_dict = {}
    word_list = string_argument.lower().split()
    for words in word_list:
        if words in main_dict:
            count = main_dict[words] + 1
        else:
            count = 1
        main_dict.update({words: count})
    return main_dict