Thank you for reaching out to us here in the Community, @risa.
I can help you add an attachment to your bank transactions.
Yes, you can add an attachment to your banking transactions, however, you'll need to make sure that the transactions was uploaded successfully.
To upload the transactions:
You can also check this article for additional reference: Upload the file to your QuickBooks Online.
After uploading the transaction, you can add an attachment to each transactions manually under the For Review tab. Here's how:
You may check this article for additional information about attachments in QuickBooks: Attachments in QuickBooks Online.
Let me know if you have any other questions. Have a good day!