Unable to merge tiles even after restarting system.
You need to close all the files and the folders linked to that projects,including the project report, mosaics etc… Otherwise you cannot reprocess.
Where and/or how do you close them?
You can open Task Manager (Ctrl+Alt+Delete) and look at the app/files you have opened. You can display them (double-click on the file) or close them (right-click) and choose ‘End task’.