Shiva -
This issue happend because an other instance is already open in the background, so BP doesn't have to read or modify on the sheet. It is caused because of improper closing of excel instance. So if you kill the process in task manager it will be unlocked for operations.
Best practice- always close instance of excel once we perform read or write operations using blueprism. So instance will be killed peacefully causing no access related issues.
Thanks,
Himavanth