11002: Can not delete the selected object(s) because one or more of them have associated features that are not reserved

An object you are trying to delete can't be deleted because other objects have associations to them. If you look in the process window, you should see messages like:

Can not delete associated object "ThisObject" (Result of Task "ThisTask"). It is not reserved.

Then try to reserve the task "ThisTask" first and any other tasks listed in the process window like this. If you do not know where in the process tree they are, just type the name in the "Filter" box above the process tree. After reserving all of these, try again to reserve the task with the loose objects.