Create a validator script in Jira Cloud

[Damla Naz Yilmaz]

If the user is already set as assignee somewhere in the project, he should not be able to transition another issue. therefore Iwould like to write a script validator. 
I already tried something like
 issue.assignee.includes(user.accountId).
But I was not succesfull.Does one of you have an idea and can help me

[Nic Brough -Adaptavist-]

I am not sure I understand what you are trying to do here.  The way I read it is:

If a user is assigned issue X then they are not allowed to do a transition on issue Y.

Is that right? 

It's quite an unusual requirement, I would expect something more like "only the assignee of Y can transition Y"

I can't tell why your code doesn't work without being clear on what it is trying to achieve.