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.
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Such problem has plugin for Worldpay.