PayPal payments may not work for various reasons, however, here are some of the most common problems that occur:
Make sure, you are using PayPal primary account.
Your product currency and PayPal currency is the same, or you've enabled the PayPal account to accept all currencies.
Make sure you haven’t blocked the unencrypted payment button - This option should be found in Paypal under Profile » My Selling Tools » Website Preferences
Make sure you set up an IPN - This setting can be found on Paypal under Profile » My Selling Tools » IPN
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