What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
I checked your form, and tried to make a test submission on your form, and got the error as "Security header is not valid". This issue happens when the information entered in PayPal API settings has issues. or some settings issue in your PayPal merchant account.
Are you putting the information by copying it from a source (like Notepad) and pasting it to the PayPal API settings fields? Sometimes, when you copy a text/word from a source it includes a " " (space) without you noticing it.
I would recommend you try to enter the information manually and see if it works this time.