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I'll share some details about the error, user2casaelegante
Here are the file extensions that QuickBooks Online supports for uploading the bank transactions:
When you get this message on the Transactions page, it could be that the .qbo file has been damaged. You'll want to re-download the file again. You can use this article as a guide: Manually upload transactions into QuickBooks Online.
The other option is to download a .csv file format and map the fields. You can follow the instructions here: Import bank transactions using Excel CSV files.
Please let me know if this helps you fix the error and upload the transactions. I'll be around to help you until a solution is found. Thanks.