A client is using manual bidding for its Shopping campaign and you decide to use target return on ad spend (ROAS) to take advantage of AdWords’ automated bidding features. Before you begin using target ROAS, you need to make sure that conversion tracking is set up for the campaign and each product group has more than 200 clicks per week a remarketing list for search ads is set up for the campaign.
Explanation: Before you can turn on Target ROAS bidding, you first need to enable conversion tracking for your Shopping campaign. For best results, each of your product groups should have more than 200 clicks per week. You can combine product groups to make sure you have a high enough number of clicks.
Read more here: https://support.google.com/adwords/answer/6309035