Inconveniently, the error doesn't appear until I click the exclamation point in the rule summary, or edit the rule, after publishing the rule. The JSON is:
{
"fields": {
"Business Analyst Signoff": { "id": "{{initiator.accountId}}" }
}
}
The error is:
Additional fields contains invalid field(s) in 'update' or 'fields' section: Business Analyst Signoff