You can't actually fail IELTS because it's not a pass and fail exam but you may fail to get the points you need to get your total to 67 points. Try to look for IELTS preparation material to prepare for the test properly to maximize your score. If you don't get enough points, you may take it again as long as you have time. Once immigration has processed your application to the point of asking for your IELTS and you send one that doesn't get you the required number of points, your application will be denied although you may apply again at a later date.
For anybody who hasn't applied yet, it is advisable to take the IELTS before you apply to avoid running out of time. That way, you can prepare for the IELTS properly and take it over until you have the number of points you need and then apply. As long as your IELTS is less than a year old when you apply, it stays valid throughout the processing of your application.