Symptoms
Client Scripts are not running on Service Portal. They run perfectly on the Standard UI.
Release
All Releases
Cause
The cause of client scripts not running on Service Portal is if the "UI Type" is set to "Desktop".
For Catalog Client scripts to run on Service Portal, the UI Type needs to be set to "Mobile/Service Portal" or "All".
Resolution
To fix this issue, change the UI Type of the client script to "Mobile/Service Portal" or "All".
Additional Information
Make sure that the scripts are using Mobile compatible API since the UI type is set to Mobile / Service Portal. If the API is not compatible to Mobile platform, the scripts will throw errors and not run as expected.
Below links have more information on what functions are supported and/or not supported on Service Portal.