That looks like a Mini 2. Have you enabled (and tested) your app for that device type?
Correction: it might actually be a later design of a Mini 1. Get the device serial from the merchant.
Yes I verified it's set for that device type, and it's signed correctly for that device type, and the min SDK is set correctly. I verified you don't have any merchant groups overriding them to install an older version. If you want to have them try the install and then do Help > Diagnostics > Send Logs I can see if I see anything.
Send me the device serial please.
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