When trying to edit a level using Level Manager in a sheet or drawing model all the levels are greyed out as shown in the image below.
When you see greyed out levels in Level Manager, open the Configuration Variables dialog box by choosing File > Settings > Configuration and search for the variable “MS_LEVEL_DO_NOT_OVERRIDE_DESIGN_LEVELS_IN_SHEET”. If the value of this variable is set to 1 then you will not be able to edit any levels in a sheet or drawing model. Simply delete that value and save your changes. After that you should see that you are able to edit levels in your sheet or drawing model. Please refer to the images below.