It’ll be some kind of issue with the configuration between the merchant’s terminal or their payment processor. Especially if it isn’t even showing up in the app. This is generally a result of Mondo being an unknown prepaid card and is normally resolved through some kind of BIN/IIN (Bank/Issuer Identification Number) whitelisting.
If you can remember the other establishment, I’d recommend reporting both in the known issues thread. If you’re feeling adventurous, you can contact both merchants to try and see if they are willing to resolve the issue on their end.