function module in update task

hello!

i am using userexit mv50afz1 in the FORM USEREXIT_SAVE_DOCUMENT. i am using transaction vl09

and thourgh this transaction i am using a bi for updating a field in transaction vl02n.

since both transactions are connected i used the fm in update task.

i have two problems:

1.when i am trying to debugg the fm i dont see the debugger go into it. i know that i should switch something on but i dont know what.

2. after i cancel the post good issue and push enter i get the following message

Express document "Update was terminated"

what exactly is it?