I have a problem with uploading csv file

I've got you covered, @Mbongenim.

Let's perform some troubleshooting steps to fix this. There are times that the browser is full of frequently accessed pages, thus causing some unusual responses. Let's log in to your account using a private browser. This mode will not save any history, making it a great place to identify browser issues.

Access your QBO account in a private/incognito browser. You may use these shortcut keys that I've listed below:

• Google Chrome: press Ctrl + Shift + N  
• Mozilla Firefox: press Ctrl + Shift + P
• Safari: press Command + Shift + N

Then, upload your CSV file. If it works, go back to your regular browser and clear the cache to delete stored site data and improve the system performance. Also, you can use other supported browsers. It could be that the one you're currently working on has a temporary problem with QuickBooks. 

I'll be adding these articles to learn more about uploading CSV file:

• Format CSV files in Excel to get bank transactions into QuickBooks
• Manually upload transactions into QuickBooks Online

Know that we're open 24/7 to help you. Just leave a comment down below if you have other concerns. Have a wonderful day.