Why can I not add an app transaction due to an error message "this contains new square customers and items"?

Hi dianamccann52,

Welcome to Community!  Connecting your Square account to QuickBooks Online makes importing your transactions effortless.  I'd be happy to assist!

With regards to the message you're receiving, I suggest taking a look at this helpful guide to manage your settings and review how your customers are set up.  This will provide a smooth transition to QuickBooks Online.  If you require additional assistance, I recommend reaching out to Square Customer Support.  

If you have other questions, feel free to reach back out.  We'd be glad to help!