Hello there, @SimonaW. I'm here to help you fix the purchase order in QuickBooks Online (QBO).

To start, we can perform some troubleshooting steps to determine if this is a browse-related issue. To do so, log in to your QBO account via incognito mode. This does not store cache and history files that might get corrupted which causes odd behavior when running web-based programs. Below are the shortcut keys you can use:

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

Once signed in, create a purchase order again and ensure that the Close box for each line item is unchecked. Then, save it and check if you can now copy it to the bill. If this work, go back to your regular browser and clear the cache to eliminate piled-up data that might cause the issue. If you still get the same issue, I recommend using another browser or device. 

However, if the same thing happens, you can also recreate the item in question. Might be that it gets corrupted that's why it shows as closed when creating a purchase order. For detailed steps, check out this article: Add product and service items to QuickBooks Online.

I'm always here ready to lend a hand if you have other concerns managing your purchase orders in QuickBooks. Have a great day ahead, and stay safe.