An advertiser attempts to enable target cost-per-acquisition (CPA) bidding but the option isn’t available. The most likely reason is that the advertiser:
has fewer than 15 conversions in the last 30 days
has fewer than 5 conversions in the last 15 days
is using Google Analytics
is using another automated bid strategy
Conversion Optimizer (CPA Bidding) is only available for campaigns that have implemented conversion tracking and have received at least 15 conversions in the last 30 days. But Google recommends that you have at least 30 conversions in the past 30 days. It also helps if your ad group or campaign has received conversions at a similar rate for at least a few days.
More to learn: https://support.google.com/Google Ads/answer/6268632?hl=en