Since you are using nested files with data sources, you need to verify the child test's properties
and if it is set to use the parent test's data or its own. The property to look for is named InheritParentDataSource.
What I suspect is that the child test is set to use the parent's data, but this data file does not have a column with the necessary data. There are two possible approaches to fix the issue:
1. Leave the child test to use the parent data and modify the data file to have a column for the child's test bound steps.
2. Disable the child's test property InheritParentDataSource
and leave the two data files as they currently are.
In addition to the above notes, please have some additional notes on data bound multi-level tests
and what behavior to expect from their execution.
I hope this will be helpful for you.