Child records (line items) can be linked when the child record is saved if the parent record already exists, including a draft saved with Save & Continue. If the parent record has not been created yet, the link must wait until it exists. See when line items get linked.

The linked field of the parent doesn't need to be included in the child's form. We recommend that you remove it to avoid confusion. The parent link is handled automatically when the records can be linked.

If for some reason you need to create the parent record and child records at the same time, you can achieve something close by enabling Save & Continue feature (previously referred to as Compute Mode)

You can also hide the linked records field conditionally to force the user to click Save & Continue first in order to see the field.