When adding Campaign Members to a Salesforce Campaign, a validation rule on the Campaign Member object (or a related object or automation) may prevent the member from being added. This can occur when the Campaign Member record does not meet the criteria defined in an active validation rule, or when an automation such as a Flow or Trigger encounters a field update error during Campaign Member creation.
Users will notice a message in Red "We couldn't add members to your campaign." followed by the error message from the validation rule. The campaign member dialog box will close and user will be presented back the screen to view all Campaign members from which they can readd the member.
You can check the validation rule under Campaign Member object to see if the Campaign Member will meet its criteria. If there are no validation rules listed under Campaign Member Object, it could be from another object or automation fails. Use Debug logs to check the failure for more accurate analysis. Items to check:
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