If you want to view or modify variable values during debugging, the program unit must be compiled 'with debug information'. There are 2 ways to accomplish this:
Enable the Debugger preference "Add debug information when compiling". Now you don't have to worry about debug information anymore, as each compilation implicitly adds it. You still need to compile a program unit at least once though.
Right-click on the program unit in the Object Browser and select "Add debug information".
The debug information goes away if you compile it without the "add debug information when compiling" preference, or when someone else compiles it without debug info.