If I use command /exp set , I will receive help message only.

duplicate of #1285.

This is not same #1285, it is not working in older versions too.

Older version of Essentials? If so, that's because this is a CraftBukkit/Spigot bug.

I'm using 1.11.2 spigot and latest version of essentials. I tried version 484 and still same problem.

@tomasmessing What happens when you type /exp?

If it doesn't say <player> has <exp> exp (level <level>)... then there is another plugin overriding Essentials.

Could it be @tomasmessing that you're using /exp to set the experience and not using /xp command?
Not sure why it takes two different commands but /exp smmmadden will show what SupaHam stated:

But if you follow up with the /xp amount player it will give that player experience points.

exp and xp appear to be different commands, although the Essentials wiki says that xp is an alias of exp.

Closing due to inactivity. If you need further assistance, feel free to respond to this issue and we'll reopen it.