Insert not permitted with template rule

Hi all,
I am stuck with the template rule where users can read and write their own data. I have used the Realm UI to deploy this rule

  "user_id": ""

But I am getting back the error

insert not permitted (status 403)

Is this correct? Where else should I look to fix because I am stumped by this

Have you set the flag to indicate that the owner is allowed to insert?

Does the document you’re trying to insert include a field named user_id?

Thanks Andrew for your reply. What I was trying to achieve was something like the below;
It appears the rules do not work with multiple conditions. Is the syntax below correct. Say:

    "agency": "",
    "%%user.custom_data.isAdmin": true