The error message in the upper-left pane <object> does not understand <script> often means that the <script> doesn't exist in the class named <object>.

For example, suppose you get the message MyPartClass does not understand otherScript

If you browse the class MyPartClass in a Script Editor or one of the Smalltalk browsers, you probably won't find the script otherScript in it.

To fix the error, you must add the script otherScript to MyPartClass. Or, you must change the script that sends otherScript so it uses a different script. Or, remove the incorrect event-to-script connection in the Composition Editor.