Spotfire: The client add-in is probably not installed on the server

When running an Automation job calling a custom extension on the Automation server "The-client-add-in-is-probably-not-installed-on-the-server" will be thrown if the Automation server does not find the custom add-in in its modules.
 

1) Intended Client set for the custom extension is incorrect
In the Spotfire server Administration console under Deployments & Packages check the Intended Client for the custom extension. The intended client can be either of these three types-
 
Spotfire Any Client: Extensions without user interface logic. Typically, an extension providing models based on the Spotfire platform.
Spotfire Professional: Any Windows Forms user interface component. Applies to the Spotfire installed client.
Spotfire Web Client: Any web client user interface component.

For a custom extension to work on Automation server the intended client should be set to 'Spotfire Any Client' when creating the extension via Package builder.If for example the intended client is say set to 'Spotfire Analyst' the the custom extension would only be applicable for the Spotfire Analyst and would not be applicable for Automation services.

2) Automation service is not updated
After the custom extension is deployed ,the Automation Server service needs to be updated. Only deploying the custom extension in 'Deployments & Packages' will not update the Automation service.It needs to be updated via the Server Administration Console - Nodes & Services - Automation Services - Click the 'Update Service' button to update the Automation server instance to get the custom modules.