Security header error on submitting form

I do still have a question on our Credit Card/PayPal payment option. I
selected the User Defined option and then had the total amount to charge
pulling from another field that is a Form Calculate field.
I connected to our PayPal account and am currently trying to test it but
can't verify that it will work correctly. I am in sandbox right now but I
thought I could still test it to make sure it works.
When I tried to submit a preview, using the info JotFOrm allows to
automatically be entered in fields, I get this message:

What does this mean? How can I verify everything is working if we want to
take credit card and paypal payments?

Generally, the error 'Security header is invalid' is displayed when the API credentials provided are incorrect in the payment field wizard. 

Could you check the API credentials entered in the payment field are correct for the Sandbox mode?