Having error in R.string R.id so on java android studio

                getServerResult.launch(
                    Intent(mContext, ChangeServerActivity::class.java)
                )
            } else if (vpnStart && !isServerSelected) {
                mContext.toast(getResources().getString(R.string.disconnect_first))
            } else {
                mContext.toast("Unable to connect the VPN")
            }
        }

This is the code and error is :"unresolved reference: string"