some skills only work on mobs, in combat against players they are not causing damage.

FAQ from our discord.

Why can I not hit other players or villagers with area spells (like Arcane Beam)?
The default configuration is PVE oriented, friendly towards villagers and other players.
Put players into different teams to allow hurting each-other.
Alternatively you can change this in Spell Engine server config (config/spell-engine/server.json5)

(Edited, pasted the wrong answer at first)