If you want to do a Master's, you will need to retake the IELTS test and achieve higher results. To get a study permit approved, CIC will be expecting that each of your four individual IELTS scores are 6.5 or higher. Sometimes it's possible to be approved with one or two individual scores at 6.0. However you have three at 5.5 - so that's going to be a guaranteed refusal.