How to default Business Unit using Object Trigger for HRHD Service Request?

Summary: We need to default the business unit for HRHD SR creation

Content (required):

Our categories are setup based on business unit and therefore the Business Unit needs to be defaulted upon the creation of an SR. This then allows the user to select the relevant categories. Without the BU set no categories are rendered in the list. We were referred to the following MOS Document - How Does The Business Unit Get Defaulted On A Service Request? (Doc ID 2765528.1) which gave us the solution of using an object trigger. I implemented this as per the screenshot

This worked perfectly for users who are admins, the business unit defaults and the categories are rendered. But when a standard employee now goes to create an SR they are given the following error: