Is there any way i can create an invoice with a past date and save it

I'll make sure you can create an invoice with a past date in QuickBooks Online (QBO), @sephra-abraham-g.

The warning you've received indicates that the date on the transaction (invoice) you want to create comes before the As of date of the inventory item. With this, you'll have to check your inventory's Starting value date. Then, ensure that the transaction's date is after the start date of the inventory item. Here's how:

1. Go to the Gear icon.
2. Under Lists, select Products and Services.
3. Find the item that you're trying to use on the transaction.
4. Click Edit in the far right.
5. On the Product/Service information page, go to the Quantity on hand section. 
6. Click the Starting value link and click Got it! when prompted. 
7. In the new window, verify the As of date. Then, update it as necessary.
8. Click Save and close. 

To know more about tracking your inventory in QBO, you can refer to this article: Set up and track your inventory in QuickBooks Online.

Once you're done, go ahead and create your invoice with a past date. Please see this article for the detailed steps: Create invoices in QuickBooks Online.

Also, I'd recommend accessing reports to instantly see your inventory status and performance. Doing so will let you know your best sellers, what’s on hand, the cost of goods, and more. You can check out this article for the complete details: Use reports to see your sales and inventory status.

Keep me posted on how it goes. Let me know in the comments if you have other concerns about creating invoices and managing inventories in QBO. I'm always around to help. Take care, and I wish you continued success, @sephra-abraham-g.