Is there anyone can answer this?

I never face something like that. Maybe you should try to do the setup once again, normally it should be upgrade the edition. Good luck!!

Select @@version is the command to give you information about sql server edition that is used. If you cannot even go to the instance have you try restarting the service?

You can't modify anywhere in the maintenance plan but you can add another step to the jobs which created by maintenance plan and add your script to it.

have you check it by running @@version command?