How to copy someone's User ID: https://support.discord.com/hc/en-us/articles/206346498-Where-can-I-find-my-User-Server-Message-ID-

Note: This is different from the user tag (for <@!312559012346331156>, the user ID is 312559012346331156, not xdk1235#7777)!

Please make sure to copy the user ID by right-clicking exactly on the username (open the profile page on mobile and copy the ID there), not somewhere nearby in the message. It's very easy to accidentally copy the message ID!