I made a sample csv file consisting of only three columns: date, amount and description. When I try to upload it to QB, drop down menu to allocate fields doesn't work

Hi, jingalbi. Welcome to the QuickBooks Community. 

I replicated uploading transactions manually on my end. I can share with you the possible reason for this and its fix. 

First, since each bank formats CSV files differently, you'll want to ensure that your CSV file is formatted correctly with the necessary columns for date, amount, and description. You can refer to this article as a guide: Format CSV files in Excel to get bank transactions into QuickBooks. 

Next, sometimes browser-specific issues can cause problems with QuickBooks Online (QBO) functionality. I'd recommend performing browser troubleshooting steps. Let's start by accessing your QBO account in your browser's Incognito Mode. Here's how:

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

Once you have opened the browser window, log in to your QuickBooks account and see if you can upload transactions and map the fields correctly. If you can, proceed to clear your browser's cache. This process will remove and fix specific issues and help the program run optimally. Additionally, you can use other devices and supported browsers to ensure everything works as expected.

When this is resolved, let me add this article about categorizing and matching transactions in QBO as a future reference: Categorise and match online bank transactions in QuickBooks Online.

We'll be here if you still aren't able to upload transactions into QBO. We're committed to offering ongoing support. Take care.