PayPal Express: Why am I getting a security error on my form?

Please advise how to fix a security error on my form, need assistance asap.

This uses to happen when the credentials used on the payment integration are not correct, make sure you're using the correct information as provided by your PayPal account, since you're using PayPal Express you must also make sure that your account is allowed to use the "Digital Goods" option, otherwise you need to turn the option on your payment integration, here is how: 

Also, make sure you click on the green "Continue" button at the bottom of the payment integration in order to save the changes successfully. 

Let us know if the problem persists.