This error message "Oops Invalid Signature" is displayed when the business has not enabled the passcode for Gift Cards. Below is the screenshot of the error how it will look like for reference:
The passcode for the Gift card can be enabled by Punchh admin by making the changes in the business settings for which you can reach out to CSM of the business or raise a ticket on "firstname.lastname@example.org".
Cockpit >> Gift cards >> Enable PassCode For Gift Cards?
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If further assistance is required, submit a ticket to Punchh Support. (For help submitting a ticket, click here)