I'm using the default code provided from Google, and I don't quite understand why it's not working. The code outputs the prompt "Please visit this URL to authorize this application: [Google login URL]." When attempting to log in with the account designated as owner of the script under the google developers console I get an "Error 403: access_denied" error with the message "The developer hasn’t given you access to this app. It’s currently being tested and it hasn’t been verified by Google. If you think you should have access, contact the developer [the email I just tried to log in with]."

from __future__ import print_function
import pickle
import os.path
from googleapiclient.discovery import build
from google_auth_oauthlib.flow import InstalledAppFlow
from google.auth.transport.requests import Request

# If modifying these scopes, delete the file token.pickle.
SCOPES = ['https://www.googleapis.com/auth/spreadsheets.readonly']

# The ID and range of a sample spreadsheet.
SAMPLE_SPREADSHEET_ID = '1vrZpCGW58qCCEfVXoJYlwlulraIlfWI2SmFXa1iPtuU'
SAMPLE_RANGE_NAME = 'Class Data!A2:E'

def main():
    """Shows basic usage of the Sheets API.
    Prints values from a sample spreadsheet.
    """
    creds = None
    # The file token.pickle stores the user's access and refresh tokens, and is
    # created automatically when the authorization flow completes for the first
    # time.
    if os.path.exists('token.pickle'):
        with open('token.pickle', 'rb') as token:
            creds = pickle.load(token)
    # If there are no (valid) credentials available, let the user log in.
    if not creds or not creds.valid:
        if creds and creds.expired and creds.refresh_token:
            creds.refresh(Request())
        else:
            flow = InstalledAppFlow.from_client_secrets_file(
                'Google_API_Key.json', SCOPES)
            creds = flow.run_local_server(port=0)
        # Save the credentials for the next run
        with open('token.pickle', 'wb') as token:
            pickle.dump(creds, token)

    service = build('sheets', 'v4', credentials=creds)

    # Call the Sheets API
    sheet = service.spreadsheets()
    result = sheet.values().get(spreadsheetId=SAMPLE_SPREADSHEET_ID,
                                range=SAMPLE_RANGE_NAME).execute()
    values = result.get('values', [])

    if not values:
        print('No data found.')
    else:
        print('Name, Major:')
        for row in values:
            # Print columns A and E, which correspond to indices 0 and 4.
            print('%s, %s' % (row[0], row[4]))

def PrintToSheets():
    main()

The error message you are getting is related to the fact that your application has not been verified yet.

As mentioned in that link all applications that access google APIs using sensitive scopes need to go though googles verification process. Normal you are given a grace period of 100 users accessing your application before your application will be locked down and you wont be able to authorize anymore users until you verify your application it sounds like you may have hit that point.

Your only options would be to go though the verification process or to create a whole new project on Google developer console and use that client id instead as the one you are using is currently locked for additional users.

Update Change

A change has been implemented on Google Developer console. You must now authorize users / testers to access your application before it has gone through the verification process. If you go to the Google developer console for your project, under consent screen you will find the following section

You can add tests users here, however you can not remove them and you can only add 100 of them so use it wisely.