The button works on my localhost Quickstart. Please remove the previous CSS and use this to Template Options -> Custom Code -> Custom CSS
@media (max-width: 767px){
  .view-course a#addtocart, .view-course .btn.btn-primary {
    bottom: 0px;
    position: fixed;
    left: 0px;
    right: 0px;
    border-radius: 0px !important;
    padding: 12px 20px !important;
    font-size: 13px !important;
    z-index: 99;
    text-align: right !important;
  }
  .view-course .price_info .splms-price-box{
    bottom: 0px;
    position: fixed;
    left: 0px;
    right: 0px;
    border-radius: 0px !important;
    padding: 0px 20px !important;
    z-index: 100;
    width: 80%;
  }
  .view-course .price_info .splms-price-box 
.splms-sale-price span{
    font-size: 24px !important;
    color: #fff;
  }
  .view-course .price_info .splms-price-box .splms-item-price span{
    font-size: 16px !important;
  }
}