I tried to charm a Sorak with charm person spell, they failed the test but nothing happens
Since the spell effect says it turns a "enemy into a ally" (and in the 5e description it says it turns any humanoid into a ally), it should have work on Soraks, right ?
charm makes it so an enemy cannot attack the caster, but it can still attack the allies of the caster.
What you said makes sense just by reading the description.
But do you know why the spell worked with a human enemy outside ruined tower, making him to attack my enemies ?
I don't know if it works on Soraks, but the Solasta version of Charm Person makes an enemy an ally - they will attack their former 'friends' and if you kill all enemies and still have the charmed person, combat ends and the charmed enemy disappears. There have been other threads pointing out how powerful this is.