Codable class doesn't conform to protocol 'Decodable'

Try to add enum Codingkeys inside struct, But it show error Codable doesn't conform Decodable.

Why am i getting conform decodable error? Should i seperate the struct ?

struct Model: Codable {
    let aps: Aps
    let link: String?
    let checkAction: String?
   
    enum CodingKeys: String, CodingKey {
           case aps ,link, alert,sound,title,body
           case checkAction = "gcm.notificaiton.check_action"
    }
    
    struct Aps: Codable {
        let alert: Alert
        let sound: String?
       
        
        struct Alert: Codable {
            let title: String?
            let body: String?
        }
    }
   

}

Is it a must to seperate the struct like below ?

struct FCMModel: Codable {
    let aps: Aps
    let link: String?
    let checkAction: String?
   
    enum CodingKeys: String, CodingKey {
           case aps ,link
           case checkAction = "gcm.notificaiton.check_action"
    }
    

}
struct Aps: Codable {
    let alert: Alert
    let sound: String?
   
    
    struct Alert: Codable {
        let title: String?
        let body: String?
    }
}