Square Integration Review Error Message. ErrorWe couldn't Add the transaction(s) due to "An application error has occurred while processing your request".

I now have the Square Integration app to allow for transaction communication and was reviewing our sales transactions so that our inventory would be more accurate. The first few transactions were fine but I got this error message one and I don't know why. Error We couldn't Add the transaction(s) due to "An application error has occurred while processing your request". Check account mapping in advanced settings and try again. My mapping is fine and the items are all seem ok - i.e. they are active etc. What am I missing?