In order to use target cost–per–acquisition (CPA) bidding, an advertiser must:
be using viewable cost-per-thousand-impressions (vCPM) bidding, with a certain number of viewable impressions in the previous 30 days
be opted into the Search Network
have a certain number of conversions in the previous 30 days
be opted into the Search Network and YouTube
Target CPA is an AdWords Smart Bidding strategy that sets bids to help get as many conversions as possible at the target cost-per-acquisition (CPA) you set. It uses advanced machine learning to automatically optimize bids and offers auction-time bidding capabilities that tailor bids for each and every auction. Target CPA is available as either a standard strategy in a single campaign or as a portfolio strategy across multiple campaigns and ad groups. To maximize results and give machine learning algorithms enough data to make informed bidding decisions, Google recommends that you have at least 30 conversions in the past 30 days.
Before you can set up a Target CPA bid strategy, your campaign will typically need to meet the following requirements:
- You’re already tracking conversion data in your AdWords account. You can do so by either setting up conversion tracking or cross-account conversion tracking (MCC accounts), or by importing your goals and transactions data from Google Analytics.
- The ad group or campaign has received 15 conversions in the last 30 days. This conversion history enables the system to make accurate predictions about your future conversion rate. So, the more data we have, the more accurate we can be.
- The ad group or campaign has received conversions at a similar rate for at least a few days.
Read more here: https://support.google.com/adwords/answer/6268632?hl=en