TypeError: Cannot read property 'id' of undefined while other code is working perfectly fine

do you know what’s wrong with the code? i use ".id "in all over my codes, but this want is not working , any solution?

case 'rob':
                            let userrob = message.mentions.members.first()
                            let targetuser =  db.fetch(`money_${message.guild.id}_${userrob.id}`)
                            let authorrob =  db.fetch(`rob_${message.guild.id}_${userrob.id}`)
                            let authorrob2 =  db.fetch(`money_${message.guild.id}_${userrob.id}`)

                            let timeoutrob = 600000;

                            if (authorrob !== null && timeoutrob - (Date.now() - authorrob) > 0) {
                                let timerob = ms(timeoutrob - (Date.now() - authorrob));

                                let timeEmbedrob = new Discord.MessageEmbed()
                                .setColor("#FFFFFF")
                                .setDescription(`:x: You have already robbed someone\n\nTry again in ${timerob.minutes}m ${timerob.seconds}s `);
                                message.channel.send(timeEmbedrob)
                            } else {

                            let moneyEmbedrob = new Discord.MessageEmbed()
                            .setColor("#FFFFFF")
                            .setDescription(`:x: You need atleast 200 coins in your wallet to rob someone`);

                            if (authorrob2 < 200) {
                                return message.channel.send(moneyEmbedrob)

                            }
                            let moneyEmbedrob2 = new Discord.MessageEmbed()
                            .setColor("#FFFFFF")
                            .setDescription(`:x: ${userrob.user.username} does not have anything you can rob`);
                            if (targetuser < 0) {
                                return message.channel.send(moneyEmbedrob2)
                            }



                            let viprob =  db.fetch(`bronze_${userrob.id}`)
                            if(viprob === true) random = Math.floor(Math.random() * 500) + 300;
                            if (viprob === null) random = Math.floor(Math.random() * 100) + 300;

                            let embedrob = new Discord.MessageEmbed()
                            .setDescription(`:white_check_mark: You robbed ${userrob} and got away with ${random} coins`)
                            .setColor("#FFFFFF")
                            message.channel.send(embedrob)

                            db.subtract(`money_${message.guild.id}_${userrob.id}`, random)
                            db.add(`money_${message.guild.id}_${userrob.id}`, random)
                            db.set(`rob_${message.guild.id}_${userrob.id}`, Date.now())
                            
                            };

Hello and welcome to the FCC community~!

Does that error happen to come with a line number, by chance?