I have a multi-select field client(s). When that field is updated, I want to update custom field Salesforce Account which is a multi select asset field so that each selection in Client(s) will have the corresponding assets field in Salesforce Account.
Note: For a single select field client, Automation to populate Salesforce Account is working perfectly. (not the rule below, a simpler rule, but my lookup is working perfectly)
Here is the automation rule:

The audit log shows that the automation does find the correct object keys;

The advanced editing JSON:

When I run this rule I get this error:

I tried "set", did not work, got an error as well.
I think there's some hidden trick out there and I can't seem to find it.