Hello sizizilly, thanks for your question! Your English is fine, Google Translate did a good job.
If using the Change HP command does not work, then you should still be able to incapacitate the Actor by applying the KO state on them. My guess is that since the HP change is not being applied through the Damage function, the plugin does not register this as Damage and the Mortal Damage flag is not turned on as a result preventing the Actor from becoming incapacitated.
Please use the temporary solution of applying the KO state for now. I will make sure to correct this bug for the next update.
Thanks,
MythAtelier Team