Plugin has a bug.
This plugin works ONLY for step by step checkout page.
Open the file PaymentInfo.cshtml and remove this code
Html.AddScriptParts(ResourceLocation.Footer, "https://js.squareup.com/v2/paymentform");
add this code
<script src="https://js.squareup.com/v2/paymentform"></script>
Or add it in main SimpleCheckout.cshtml or SimpleCheckout2.cshtml file
Plugin loads PaymentInfo.cshtml using Ajax. but nopCommerce doesn't add this script in the header of the page.
Such problem has plugin for Worldpay.