When I go to SQL Server Management Studio -> Management -> Maintenance Plans -> Right click on the maintenance plan that I want to execute but the option is greyed out.
I had the same problem. Turns out it was an execution window that was buried. It doesn't show as an "open window" when you look at the taskbar at the SSMS icon, so it's deceptive. ALT-TAB your way through the windows and you should find it.
The open windows was: