When clients get this error, that means they've logged into APIC with a different email address or userID than the one that was used to submit the order. This often occurs when their browser has other email credentials saved rather than the ones they normally use, and logs them in using the incorrect credentials.
Please message the Order Number to an agent or Team Supervisor with admin access who can go into the Order History and provide you with the email address or userID that was used to submit the order.
Provide the correct email address/userID to the client and offer to assist them to login with that ID.
If they do not know the password to it, they will need to use the Forgot Password function.
If the browser keeps auto-logging them into the wrong ID, you will need to perform a cache clear (and/or cookie clear) and ensure auto-fill is disabled.