Using Solver to Maximize a value

I'm trying to maximize cell D58 i.e. "NPV " by having the add-in solver change the number of units in cells L27:L32 using the constraints in cells J27:J32 and H27:H32.

In other words, when solver is changing cells L27:L32 cell L27 should be between H27 and J27 or it should go to 0, (and so on for cells L28 to L32.

The other constraint is that Cell M33 should be equal to or less than Cell N33.

I keep getting an error message and when I try to use only one of the constraints i.e. minimum project size or maximum project size, the number of units don't fit within the minimum and maximum range.

Does anyone know how to use solver for this?

I would greatly appreciate it and I've attached the file.

Thank you in advance - Eliot