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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.