@KasiaM40 You might be sure to enable the FilePublisher so that you can see the exception in the RuntimeLog.txt. I imagine there is an exception happening that would get written to the logs that might be helpful to see.
@KasiaM40 I apologize for the delay Kasia. I missed your reply somehow. That seems to indicate that you haven't defined the Workbook yet. You must first open your workbook before you can run any methods against it. Can you provide screenshots of your automation?