Paypal issue

Good morning all,

I get an error when trying to use the PayPal feature in Knack.

An error occurred when initiating the PayPal transaction: ACK Failure: Security header is not valid

Can someone clarify what fields corollate with each other.

So in knack

username = account

password = client Id

Signature  = secret.

I have tried various combinations and keep getting this error.

Any help please.