You can select a goal for your campaigns:
Link clicked / Form submitted: count the number of interaction through click rate or form submitted
In case you can't click on the button ‘Enable auto-optimization’ and got a notification as below:
You just need to ignore it and proceed to save your campaign.
For your information, 'Enable auto optimization' is an A/B testing function. When you create more than one version for your campaign and select this feature, the system will automatically learn the campaign for a selected period of time to find out which version works best for you (has the highest interaction). Then it will keep that version enabled while disabling the others.
Otherwise, if you only have one version of the campaign, just choose ‘Link clicked/ Form submitted’. Our system continues to count Impression and Interaction until you manually disable it.