Using Solver to achieve target

The problem is with the constraints which can only be imposed on the variable cells you specify. If I understand your intention correctly, you want to change the values of D12:F14 in such a way that G9 becomes 900.

Note that, even if your syntax would have been workable, this wouldn't be possible for two reasons. One, G9 is already 900. Two D15:F15 can't remain 0 if values are entered in D12:F14. I deleted all the constraints you had entered and added D12:F14 = int so as to allow only integers.

However, it seems to me that you intention was reaching much further - further than Solver can go. You may require three Solver actions to achieve what you want.

1. Change D12:F12 (constraint = int) to make G6 = 250
2. Change D13:F13 (constraint = int) to make G7 = 350
3. Change D14:F14 (constraint = int) to make G8 = 300 (no change)

This effect can't be achieved with one Solver action.