Hello @[deleted], you stated you exported the customer's list as CSV, and the try to import transactions. The issue here is that your customer's list, will only include the information about your customers, it won't include the transactions linked to the customers.
Sadly, there is no way to transfer all your data from QuickBooks into Express Accounts, because the formats are different.
If you are able to convert the data to CSV, you should be able to load transactions directly to the journal in Express Accounts. But, you must be sure the data includes the require fields (Journal, Reference, Description, Account, Debit amount and, credit amount).