You could get to PPL, Hour building, ATPL ground exams stage without a class 1. However a class 1 is requred for CPL issue, and valid class 1 audigram is required for IR. Although you can do the CPL without the class 1 as it is dual, it is required for license issue, consequently you could not take the MCC.
So in response to your question you could not gain an fATPL then fail the medical. You could fail the medical which would disallow you to reach fATPL.
If you are talking about a Class 1 renewal failure then this could be a possibility, however I note that you do not have a Class1 yet. Best advice is take class1 first.