Copilot instructions for comparing two columns in an Excel file

I've been struggling to create an agent that will compare two columns in an Excel file. I've tried it about ten different ways with no luck. Copilot simply will not follow the instructions. I do have a Copilot for M365 license.

I'm trying to compare users in a third-party app to our M365 user list for an audit. Both lists have been exported to CSV and compiled in an Excel file. Column A and B only contain user names. Column A has ±400 users and Column B ±2500 users. I've instructed the agent to look at Column A and compare it to Column B and return a list of users from Column A that don't appear in Column B. I realize this can be done in Excel with a formula, but I figured this would be fairly easy for Copilot to figure out. Evidently not. Any help with usable instructions would be appreciated. I've written the instructions several ways and generally get the same results.