Hi,
I invite you to open the file plugins/hikashoppayment/paypalcheckout/paypalcheckout_end.php
There, the first line of code (after the first few lines of the header) is:
<script src="https://www.paypal.com/sdk/js?<?php echo http_build_query($this->params); ?>" data-partner-attribution-id="<?php echo $this->bncode; ?>" <?php echo $this->extraParams; ?>></script>
So, as you can see, the plugin does provide the proper URL for the SDK file and its actually impossible for it not to.
This leaves only one potential issue: You have an extension on your website which processes the HTML on the page and wrongly modifies the src attribute of that script element even though it should not do anything with it.
It's probably caused by a cache or optimization or SEO extension or the likes.