PayPal has depreciated the PayPal Standard API which requires the use of their IPN (Instant Payment Notification) system which has been awful for as along as I can remember (and often causes orders to not complete since the IPN never gets sent). I would enable the PayPal Express gateway in Store > Settings > Payments Methods then Add New Payment Method which should give something that looks like:
PayPal Express Checkout requires an API Username, Password, and Signature. These are different from your PayPal account details. You can obtain your API details by logging in to your PayPal account, and clicking Profile > My Selling Tools > API Access > Request/View API Credentials > Request API Signature.
Once you have enabled the gateway make it the default, if you have hardcoded the original Gateway Short Name in any of your templates you will need to change that to the
paypal_express short name and then you can disable the PayPal Standard gateway.