Just yesterday we started having customers tell us we had this issue.
I searched Google for an answer to this and I found the following article: https://simplify.desk.com/customer/portal/articles/2085428-invalid-or-missing-timestamp-in-jws-request-message
I don’t know how to change a server timestamp, and I don’t know how to find out what the timestamp should be from the payment Simplify.com?
I already put the website in Support Access Mode.