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;
}
}