The default value you are trying to set for the field is not within the range of values that have been defined by the domain assigned to the field.
Review the domain that is currently assigned to the field. You can either choose a new default value that falls within the range of the domain, or you can modify the valid domain values. If using a range domain, you can modify the range using the Set Value For Range Domain tool. If your domain uses coded values, you can add a new coded value using the Add Coded Value to Domain tool.