One of the possible reasons why you're getting this error may be on your bank's end. I'm here to help you sort this out, Marian.
You can contact your bank to check if they have ongoing issues or possible maintenance that prompts you not to manually upload your bank transactions. However, if the problem persists, I recommend contacting our support team to find out what's causing it.
- Sign in to your QuickBooks Online company
- Select (?) Help.

- Go to the Search tab, then type in Contact us.

In case you want to take a look and review the process of uploading your bank statement, I've added this handy article for your guide: Manually Upload Transactions into QuickBooks Online.
Additionally, you can learn how to review categories after transactions are automatically downloaded. Go through this article for more details: Categorize Online Bank Transactions in QuickBooks Online.
We also have our QuickBooks Live Expert Assisted team, dedicated experts who will help you clearly understand your business's financial status. Ensuring you're always informed and empowered to make the best decisions for your company.
If you need further assistance when uploading your bank transactions, you can reply below. I'm always here to help.