client.on('message', async message => {
    if (message.content.startsWith(prefix + "rolelist")) {
        const Role = message.guild.roles.cache.find(role => role.name == "TEST ROLE");
        const Members = message.guild.members.cache.filter(member => member.roles.cache.find(role => role == Role)).map(member => member.user.tag);
        message.channel.send(`Users with ${Role.name}: ${Members}`);
    };
});

由于您使用的是 V12，因此忘记将 cache 添加到 message.guild.members 和 message.roles.

You forgot to add cache to message.guild.members and message.roles since you are using V12.

另外，你使用的 find 函数有误.

Also, you were using the find function wrong.

你不能这样使用它:

member.roles.cache.find(testRole)

这是你应该如何使用它:

This is how you should use it:

members.roles.cache.find(role => role == testRole)