Budget Allocation
If you selected Automatic Budget Allocation, you will have an additional Creative step in the fifth step of building a line item, Buying Strategy.
You will see a Creative Type section that is greyed out. This is because Creative Type is selected at campaign level and applies to all line items under that campaign
Next, select the creatives want associated with your ad
Finally, you will see your buying strategy is grayed out. This is because the Automatic Budget Allocation will handle the buying strategy piece