I have an open PO but I forgot to copy to bill, then separately I add a bill wich is the one that I have in open PO. so my question is how to link these two?

Thanks for visiting the Community today, @sistemas1. Since you're unable to link the purchase order (PO) to an existing bill, you can delete the bill first. Then, copy the PO to the bill. 

Here's how:

1. Go the Expenses tab, then select Vendors. 
2. Locate and select the vendor to view the transactions. 
3. Find the Bill and click it to open. 
4. Click More at the bottom. 
5. Select Delete. 

Once done, go back to the vendor's transactions list. Then, follow the steps to copy the PO to bill. Here's how: 

1. Locate the Purchase Order. 
2. Under the Action column, select Copy to bill in the drop-down menu. 
3. Then, click the Save button in the Bill window. 

To know more about applying a purchase order to a vendor transaction, read through this article: Create purchase orders in QuickBooks Online. 

For future reference, here's an article to help you learn about entering and paying bills in QuickBooks Online. 

Please let me know if you have additional questions. We're always here to help. 

Is there anyway to add the PO# to the bill after it's been created without deleting the bill?

Thanks for joining us here in the Community, mbishop20.

I'm happy to provide some info about adding purchase order numbers to bills in QuickBooks Online.

Just to make sure I'm on the same page, do you need to add only the purchase order number, or do you need to attach the purchase order itself? 

If you only need to add the number, you can do so on an existing bill by:

1. Hovering over the Expenses tab and then select Bills.

2. Locate the bill you'd like to add the PO number to and click the drop down arrow to the right and choose View/edit bill.

3. From there you can enter the purchase order number in the Memo section.

4. Click Save and new or Save and close when you're finished.

If you need to attach the actual purchase order to the bill, the following article provides additional info about adding purchase orders to expenses, bills, or checks in QuickBooks Online.

Please don't hesitate to reach back out if you have any other questions. I'll be here to lend a hand.

I need to attached the PO itself. I have multiple bills that I'd rather not delete and re-enter.

Good morning, @mbishop20. 

Thanks for reaching back out and giving us some additional information. 

Once the bill is closed, it's considered finalized and can't be modified to include the purchase order. 

I recommend deleting and re-entering the bills with the steps listed on this guide to properly add the purchase order to it: 

Add purchase orders to expenses, bills, or checks in QBO

Feel free to come back if you have any further questions about purchase order, bills, or other features in your QuickBooks Online account. I'll be right around the corner if you need me again!