Paypal Express issue

Paypal_express: Security header is not valid

Anyone know what this could be caused from?
I put in my api username and api password from the paypal website
I used my api cert fingerprint for the signature. Should I be using something else?

In Invoice Ninja on Settings > Online Payments edit the PayPal settings and disable test mode.

Test Mode has been disabled, still get the error

I’m not sure, you may want to check your PayPal credentials,

I’ve seen this error reported quite often, in all cases the problem was using either live credentials with test mode enabled or visa versa.

Can you confirm that the field that says signature is indeed the thumbprint from my paypal cert?

Thank you for the clarification. It turns out I had a paypal certificate enabled.
I had to remove that and go with the signature option.

It’s working now!

Thanks for letting us know, glad to hear it’s working!