Your shopkeeper must be level 6 to participate in City Raids.
Steps in raids are also locked by shopkeeper level. This is to prevent players from exploiting the feature by joining cities that are much higher than their level.
If your shopkeeper can only participate in a portion of the raid steps, you will receive awards for the available steps, but not the locked steps. You must participate at least once during the event to receive prizing.
Example: Your shopkeeper level allows you to participate in 3 steps of the City Raid. Your city can complete up to 6 steps. If you participate in at least one steps, you will receive all the rewards up through step 3.