2 things you did wrong (or that im aware of)
1 (MAIN ISSUE): When you tried to .get a user, it can only accept an ID, not the username & tag. Try using
client.users.cache.get('your id, for example 2983190921391')
.
2: You have to use await because it takes time fetching the user. Put it in an async function and use
let user = await client.users.cache.get('your user id again')
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