Thanks for reaching out to the Community, @bhuwan.kotabagh.
May I know which specific bank you've connected to QuickBooks Online (QBO)? Also, may I know the error message that you're getting? Any additional information will be much appreciated.
In the meantime, try signing into your QBO using a private browser (incognito). This will help us check if this is a browser-related (cache and cookies) issue.
Once logged in, let;'s try to upload your bank transactions again to double-check. Here's how:
If it's successful, go back to your default browser and perform a clear cache to refresh the system. However, if the issue persists, try using other supported browsers.
For more insights about uploading bank transactions in QBO, consider checking out this article: Manually upload transactions into QuickBooks Online.
After that, you can now proceed with the reconciliation process. Visit this article for future reference: Reconcile transactions that are older than your opening balance in QuickBooks Online.
Please let me know if you have any additional questions in the comment section below. I'll be here to help. Have a great day!