Issues with addRole to user when hitting a certain level - discord.js

Question

I'm trying to add a function to my discord bot xp-system by adding a user automatically to a role when hitting a certain level.
I have been trying all kind of solutions I can think of but none seem to work.
Have tried:
addrankup.addRole(fullmember).catch(console.error);
let addrankup.addRole(fullmember).catch(console.error);
addrankup.addRole(fullmember.id).catch(console.error);
Expected when users hit a set level to automatically get the rank:
let xpAdd = Math.floor(Math.random() * 5) + 10;
console.log(xpAdd);
if (!xp[message.author.username]) {
xp[message.author.username] = {
xp: 0,
level: 1
};
}
let curxp = xp[message.author.username].xp;
let curlvl = xp[message.author.username].level;
let nxtLvl = xp[message.author.username].level * 200;
xp[message.author.username].xp = curxp + xpAdd;
if (nxtLvl <= xp[message.author.username].xp) {
xp[message.author.username].level = curlvl + 1;
message.channel.send("Level Up!").then(msg => {
msg.delete(5000)
});
}
if (curlvl <= xp[message.author.username].level) {
xp[message.author.username].level = "2";
let addrankup = message.mentions.members.first();
let fullmember = message.guild.roles.find(`name`, "Full Member");
addrankup.addRole(fullmember.id).catch(console.error);
message.channel.send("You have received a new role!").then(msg => {
msg.delete(5000)
});
}
fs.writeFile("./xp.json", JSON.stringify(xp), (err) => {
if (err) console.log(err)
});
But instead this is what the console tells me:
(node:8460) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'addRole' of undefined
at Client.bot.on (E:\#DeathKillerNOR Discord Bots\DeathKillerBot\index.js:125:15)
at Client.emit (events.js:187:15)
at MessageCreateHandler.handle (E:\#DeathKillerNOR Discord Bots\DeathKillerBot\node_modules\discord.js\src\client\websocket\packets\handlers\MessageCreate.js:9:34)
at WebSocketPacketManager.handle (E:\#DeathKillerNOR Discord Bots\DeathKillerBot\node_modules\discord.js\src\client\websocket\packets\WebSocketPacketManager.js:103:65)
at WebSocketConnection.onPacket (E:\#DeathKillerNOR Discord Bots\DeathKillerBot\node_modules\discord.js\src\client\websocket\WebSocketConnection.js:333:35)
at WebSocketConnection.onMessage (E:\#DeathKillerNOR Discord Bots\DeathKillerBot\node_modules\discord.js\src\client\websocket\WebSocketConnection.js:296:17)
at WebSocket.onMessage (E:\#DeathKillerNOR Discord Bots\DeathKillerBot\node_modules\ws\lib\event-target.js:120:16)
at WebSocket.emit (events.js:182:13)
at Receiver._receiver.onmessage (E:\#DeathKillerNOR Discord Bots\DeathKillerBot\node_modules\ws\lib\websocket.js:137:47)
at Receiver.dataMessage (E:\#DeathKillerNOR Discord Bots\DeathKillerBot\node_modules\ws\lib\receiver.js:409:14)
(node:8460) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated
either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:8460) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

Answer 1

The error tells you that addrankup is undefined which in turn means that message.mentions.members.first(); returns nothing. My guess is that instead of
let addrankup = message.mentions.members.first();
you're looking for
let addrankup = message.member;