Please make sure you are using the right API settings and that you are using the API settings of the selected account type (Normal / Sandbox).
You can find this in the config.php, open the file
and try to find something called $sandboxFlag make sure that this is set to true when you are in sandbox mode and to false when you are in live mode
$sandboxFlag = false;
Make sure your API credentials are set correctly, more info about how to find them can be found over here
Make sure your Paypal account is a business account, else you won’t be able to create any checkouts. More info about a business account can be found here
Make sure that you are not using a comma ( , ) for decimal prices, instead you should use a dot ( . ) for decimal prices. So e.g. 5.99
Make sure you are using the latest version of the Premium plugin and Droppy itself.