You can clear any doubt by making a call directly to the ExpressCheckout endpoint using your API credentials. If it fails then there is something in your API credentials but if it is successful like I think then something in your integration/module is not passing the correct API credentials to PayPal.

Just run the html form below into your browser and replace the API credentials with yours. Then click the button "SetExpressCheckout" at the bottom and see the result, you can see that you are making a basic SetExpressCheckout API call towards the live endpoint https://api-3t.paypal.com/nvp:

<form method="post" action="https://api-3t.paypal.com/nvp">
API Username: <input type="text" name="USER" value="uk-3xxxx">  <br>
API Password: <input type="text" name="PWD" value="D25xxxxx">  <br>
API Signature: <input type="text" name="SIGNATURE" value="A7yWAlFxxxx">  <br>
Version: <input type="text" name="VERSION" value="204"> <br>
Paymentaction: <input type="text" name="PAYMENTREQUEST_0_PAYMENTACTION" value="sale"> <br>
Amount: <input type="text" name="PAYMENTREQUEST_0_AMT" value="7.50"> <br>
Item Amount: <input type="text" name="PAYMENTREQUEST_0_ITEMAMT" value="7.50"> <br>
Currency: <input type="text" name="PAYMENTREQUEST_0_CURRENCYCODE" value="USD"> <br>
ReturnURL: <input type="text" name="returnUrl" value="http://www.mywebsite.com/success.html"> <br>
CancelURL: <input type="text" name="cancelUrl" value="http://www.mywebsite.com/cancel.html"> <br>
SolutionType: <input type="text" name="solutiontype" value="Sole"> <br>
<br>
<input type="submit" name="METHOD" value="SetExpressCheckout"> <br>
</form>