How are creatives assigned to a line item?
While setting up a new creative for the advertiser, assign the line item and click Save
Select the creative to assign in the line item’s creative section
In the line item’s settings, search and select the correct 1×1 pixel
Bulk upload a creative spreadsheet in the advertiser’s creative section
In Display & Video 360, you first add creatives to an advertiser, then select specific creatives for each line item. Click Assign creatives in the Creatives section of the new line item page. Click the checkboxes beside the creatives you want to use for the line item. Click Done.
When you’re done setting up the rest of your line item, click Save.
Read more here: https://support.google.com/displayvideo/answer/2696848