Saving as CSV file not possible from Barclay Bank American Aviator Mastercard, now what?

Yes, you'll want to download it via Excel and save it in CSV file format, wkreitler-9-line.

When you download your bank transactions, it may ask you where to save the file and what file format you'll want to save it. When prompted with the Save As window, you can select CSV (Comma delimited) under the Save as type drop-down. You can use the screenshot below as a reference. 

Then, you can start importing them into the system. You can use this link that will walk you through the detailed step: Add older transactions to QuickBooks Self-Employed. Once done, you can then categorize them accordingly. 

Let me know if you have other questions. Take care and have a great day!