Hi Lisa -
This question really doesn't have anything to do with usability. This was probably a better question for the "General help" discussion.
In any case, Moodle 1.9 has been out of support since July 2012. It will be difficult getting help with it any more.
Looking at the file and line you have mentioned in the code repository, there is no "iscomplete()" function call. I searched that entire file and could not find an occurrence of that call anywhere. I also did a search of the entire 1.9 codebase and could not find any occurrence of the "iscomplete" function.
My guess is that you have made customizations in your code. And somehow you have lost some of those customizations. If you have another site with the same code at that line number, and it is working, then it must still have the customizations you need.
Looking at the attachment you provided, the file has definitely been customized. It looks like it has the old "Activity locking" hack added to it (probably this - https://docs.moodle.org/32/en/Development:Activity_Locking#Table_of_Versions). The function that hack provided has been part of Moodle since some version in Moodle 2. It would be best if you upgraded to a the latest (3.2).
Hope this helps.