您应该使用type等于6。如Discord文档中所述，选项类型应该是数字。

SUB_COMMAND 1   
SUB_COMMAND_GROUP 2 
STRING 3    
INTEGER 4
BOOLEAN 5   
USER 6  
CHANNEL 7
ROLE 8  
MENTIONABLE 9
NUMBER 10
ATTACHMENT 11

使用您的代码实现：

const { Client, CommandInteraction } = require("discord.js");

module.exports = {
    name: "test",
    description: "mention a user",
    type: 'CHAT_INPUT',
    options: [
        {
            name: 'user',
            description: 'mentions a user',
            type: 6,
            required: 'true'
        }
    ],
    /**
     *
     * @param {Client} client
     * @param {CommandInteraction} interaction
     * @param {String[]} args
     */
    run: async (client, interaction, args) => {
        interaction.followUp('yes');
    },
};