Type 'WatchManager' does not conform to protocol 'WCSessionDelegate'

I try use WatchManager.swift in exists project.

import Foundation
import WatchConnectivity

class WatchManager: NSObject, WCSessionDelegate {


    func session(_ session: WCSession, activationDidCompleteWith activationState: WCSessionActivationState, error: Error?) {

    }

    func sessionDidBecomeInactive(_ session: WCSession) {

    }

    func sessionDidDeactivate(_ session: WCSession) {

    }
}

but I have trouble on build my project

Swift Compiler Error Group - Type 'WatchManager' does not conform to protocol 'WCSessionDelegate'

Also in issue navigator i see error

Candidate has non-matching type '(WCSession, WCSessionActivationState, Error?) -> ()'

I already tried:

• remove derived data
• clean project
• clean builds folder
• reload project from git
• copy file from another project
• create new file and rename it
• build project on another mac

but i get this error everytime

BUT! If I will create new project and add watchOS target and create new file with this code - project will build successfully.

Why? How i can try to add WCSessionDelegate in my exists project?

I use Xcode9.2, swift4.0

Oh... Thank @Sameer for answer

WCSessionDelegate has required method

public func session(_ session: WCSession, activationDidCompleteWith activationState: WCSessionActivationState, error: Error?)

but my project there is my custom class with name Error

that's why the compiler produced an error that does not conform to protocol