You can’t create or modify a transaction with a date that comes before you started tracking quantity on hand for the following

I'd be happy to shed light on the error message you received when entering a purchase order into a bill.

The error message means that you cannot date a transaction of a particular inventory item before its start date. Note that you are asked for the "As of date" when entering that type of item.

You can adjust the start date of the item if it's incorrect. Here's how:

1. Go to the Gear icon and choose Product and Service.
2. Click Edit on the inventory item.
3. Click on the Starting value in the Quantity on-hand section.
4. Read the prompt message and choose Got it.
5. Change the As-of date and click Save and close. After the change, you can edit your transaction again. 

I'm adding these articles you can use in the future: 

• Adjust inventory quantity on hand in QuickBooks Online
• Use reports to see your sales and inventory status

It's my pleasure to help you if you have other inventory or invoice concerns. Just leave it in the comments section below or create a new thread. Keep safe.