I don't know how did that happened but I'm using onepagecheckout
Website: https://femascu.net/
Product to add to cart: https://femascu.net/femascu-leather-watch-8401.html
I don't know how did that happened but I'm using onepagecheckout
Website: https://femascu.net/
Product to add to cart: https://femascu.net/femascu-leather-watch-8401.html
I have verified your one-page checkout page HTML code by Inspect Element and found that one of the Checkout Page CSS showing or set as "Display: none".
You can check the below screenshot for your reference. 
You can resolve this issue by following steps.
1] Removing or commenting on the below CSS class code from the checkout CSS file. CSS File Path : https://femascu.net/static/_cache/merged/1784163dac242cad636193ba20c62a2e.css #checkout-payment-method-load .payment-method-content .actions-toolbar .primary .action.checkout { display: none; } 2] Modifying the below CSS class code from the checkout CSS file. #checkout-payment-method-load .payment-method-content .actions-toolbar .primary .action.checkout { display: block; } Finally, run the below Magento cmd to deploy static content again.
php bin/magento setup:static-content:deploy