Setting an issue value based on a {{lookupIssue}}'s value

I'm trying to take a value from a free form text field (single line) and populate a drop down field based on what the user typed in.

However, there are 100+ different options, so I was trying to use the power of Jira and automation rules to make this maintanable for non-programmers.

I thought I could build an automation rule that uses "Lookup issues" to find an issue, then copy the field value from the looked-up issue to the current issue.

Here is my automation rule: 

Everything works except the last action to edit the current "Customer" field.

Any ideas what I'm doing wrong?

Thank you~!