I have a feeling you are running into the ‘access denied’ error because you are trying to kill the excel.exe process across ALL users. You should instead only kill the process for the current user - see example of how to do this here: How to kill the excel process which is running by current user - #4 by aksh1yadav
You could make the robot have local admin access on the machine which would allow it to kill the process for all users on the machine - it is a much better idea to properly deal with the issue by killing only the process you want though, as killing all process for all users could have unintended consequences