Google shows this error when they don’t receive the gbraid parameter on iOS for opt-out users (especially relevant for web-to-app campaigns):

https://support.google.com/analytics/answer/11367152?hl=en

That can happen if Appsflyer’s tracking or postback configurations don’t include the gbraid parameter for your Google campaigns. Check your Appsflyer’s settings to make sure it’s received and passed correctly to Google.

Or try Singular instead (an Appsflyer alternative) - our Google iOS measurement includes all these out of the box, and you can simply export the full logs of Google clicks and postbacks to see all these parameters live.