The REFBUILD task is a startup autotask. When this is the case, the subschema load module for sub DBSUB01 needs to found in the DEFAULT dictionary.
To determine your DEFAULT dictionary, issue the following at Enter Next Task Code:
DCMT D DBN
This will display the CV's dbname table.
On the first page of the display, you will see the default subschema mappings.
You should see a line that looks like this:
SUBSCHEMA IDMSNWK? MAPS TO IDMSNWK? USING DBNAME xxxxxxx
Where 'xxxxxxx' is the name of your DEFAULT dictionary.
This is where the subschema load module must reside.