This error occurs when trying to open a dialog box from a user generated ArcView extension:
"Error in <script_name> at ####
A(n) Nil object does not recognize request Open"
The Avenue script that opens the dialog box in the extension uses the AV.GetProject.FindDialog request to locate the dialog box. Since the dialog box is part of the extension, rather than the project, ArcView can not locate the dialog box.
Use the AV.FindDialog request to locate dialog box. For more information, refer to the ArcView Help topic Accessing Dialog.