Macro stops working after I alter a cell

[madhatter40]

I have a basic macro that basically clears contents and changes the cells to a specific color and focus. I have uploaded a basic version of this workbook.
To start I have a drop down menu located in cell G2 on ScrapCalculator tab. When you select the item it works just fine. My problem is if the user selects
some other cell and changes it breaks the macro and now it doesn't work. An example is: if you select D10 and input say 10 then the macro stops. Can
anyone tell me why and what can I do to stop this? I know it has stoped working because it no longer flashes and in my original workbook it won't change anymore.

I tried to lock the cells, but the VB freezes because it needs to access these cells to change formats.