How can I automatically filled a multiple user pick field based on child issues assignees

[Umar Maroof]

In Jira cloud I have epic as a parent under epic there is few tasks ..I want whenever these tasks assigned to someone those assignees will copy to an field which is present in epic and when the assignee is unassigned those users will automatically remove from that field and when new user assigned it will add to that field

until now what I did I construct a rule 

Trigger field value changed =assignee

condition=issue Types equal to tasks

Branch for parent(epic)

advance field edit

{
"update": {
"customfield_10195": [
{
"add": {
"id": "{{childIssue.fields.assignee.accountId}}"
}
}
]
}
}

but it give following error

MDP-325 (Field 'customfield_10195' cannot appear in both 'fields' and 'update')

this is the first step I am trying to do to add the assignee of child to epic field

[Trudy Claspill · Accepted solution]

Hello @Umar Maroof 

Is this related to your other post here where the above question is part of the dialog there?

https://community.atlassian.com/t5/Jira-questions/set-security-for-parent-issue-so-assignee-of-child-issue-can/qaq-p/2324627