Coupon Section Not Opening at Checkout

After the most recent updates the coupon field at checkout will not open when the link is clicked. We rolled back after the 8.6.0 update and the dropdown began to work again (8.5.2). Today, we decided to try the 8.6.1 update and it gave us the same results. I narrowed it down to the Siteground Optimizer plugin. We never had this issue in the past.