Your condition checks if there is more than one user tagged, but you can check if there is any tagged users with
if (message.mentions.users == true)
Second problem is that users is a collection so you have to use .size instead of .length
if (message.mentions.users.size > 0)
Both are good.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…