To track your ad set over iOS devices, you have to enable SKAdNetwork. An app is limited to 20 simultaneously live ad sets for measurement.
If you have more than 20 live ad sets for the same app, you can't publish more ad sets with SKAdNetwork tracking enabled. You need to make some SKAdNetwork Campaign IDs available by detaching the SKAdNetwork Campaign ID from it. If you keep the previous ad set running, it continues to deliver the ads but you don't get iOS measurements for the ad set anymore.
ℹ Note: There's a 48-hour cooldown period for an SKAdNetwork Campaign ID before you can safely use it in another ad set. If you use reuse a campaign ad set before the 48-hour period is over, iOS attributes conversions happening in the old ad set to the new ad set due to a delay in reporting imposed by Apple.
Detach an SKAdNetwork Campaign ID from a running ad set
Detaching a SKAdNetwork must be done via the Snapchat Ads Manager.
See SKAdNetwork Set Up and Campaign Management in Snapchat's help on how to detach a SKAdNetwork Campaign ID.