A pleasant day to you, @farhan_ciit.

You don't always get a warning message on invoices when you oversell an item in QuickBooks Online. To be notified if you are out of stock already, you'll need to set a reorder point. This process will give you alerts whenever the stocks are low or no longer available.

To do that:

1. Click on Sales at the left pane. 
2. Hit on Products and Services.
3. Find the product you want to have a reorder point.
4. Select Edit in the Action column.
5. In the Reorder point field, add the threshold when you should reorder more stock.
6. Select Save and close.

After that, you can now check if you have low stock or out of stock items.

Here's how to do that:

1. Hit on Sales.
2. Go to Products and Services.
3. At the top, you’ll quickly see if you have low stock or out of stock items.
4. Select Low stock or Out of stock to see those products.
5. If you have low stock or out of stock alerts, here's how to reorder inventory with QuickBooks.

To fix the negative inventory made, you can resolve it by following the steps presented in this link: Fix negative inventory issues in QuickBooks Online.

I'm also adding here some links about adjusting quantity on hand and entering inventory products in QBO for reference:

• Adjust inventory quantity on hand in QuickBooks Online
• Add inventory products in QuickBooks Online

Drop me a comment below if you have any other questions about inventory in QuickBooks Online. I'll be happy to assist you further. Take care always!

@farhan_ciit 

@BettyJaneB 

"Hi, Found one problem e.g I have 20 items in my inventory and when I make an invoice I entered 30 items.It's not giving me any warning or prompt that you are out of stk"

Unfortunately, such a warning sign or prompt is currently not available. But it's requested feature for a few years now.

But once you enter the product/service item, if you hover your mouse to the QTY column, it will give you quantity on hand for that item. See the screenshot below for reference.

Hope this helps!