For email capture and Send-to-Messenger promo pages, you can view your campaign's conversion rate on your Dashboard.
(Conversion rate is calculated as Promos Claimed / Unique Views)
For Click-to-Messenger campaigns, conversion rate is not available on the Dashboard, and neither is the Unique Views counter. This is due to the fact there is no landing page to install a tracking code.
You can still easily calculate your conversion rate and assess how successful your campaigns have been.
You will need to go into Facebook Ads Manager to find the total stats on Impressions/Interactions with your ad.
Then you can match these numbers with the total coupons claimed, from your Dashboard. This will allow you to calculate conversion rate in terms of Coupon Claims vs Ad Views, or Coupon Claims vs Ad Clicks.