This is a wrong question.
You are trying to add data to a cell if the cell has no data. This is what setting a formula to set a value in a cell is attempting. As you might imagine, that's not going to work.
What you really want to do, is format the cell based on the data within it (in this case a blank). You've started down the road of setting values when you should really start down the road of how to do conditional formatting. Unfortunately, whilst I can get you close to what you want and in theory it ought to work, I can't quite get "N/A" to appear in the cells. I'll explain the process though as I've already tried to do this and might as well share how close I got.
Select one or more cells. Click on Conditional Formatting (I'm assuming a current version of Excel which if you have a subscription model you should have). What you want is a bit unusual so don't select any of the commonly suggested conditional formats, instead go straight to "New Rule" at the bottom of the list.
This brings up a dialogue where you can set formats as you wish. Let's do this in stages to avoid mistakes as there are a couple of places where you could easily go wrong. First select "Use a formula to determine which cells to format" as the rule type.
Find a blank cell in your spreadsheet that will always be blank. In this example, I'm just going to use $A$2.
For the formula enter the following: ISBLANK($A$2). Do NOT click Okay yet! You still have to define the formatting.
Click on the Format box or option to bring up the choice of how to format the cell. Go to "Custom" at the bottom of the list and enter the bolded following in the field at the top and nothing else: "NA" . Note that the quote marks are to be entered (which is why I have bolded them as well).
Click Apply and you should see NA appear in the selected cells. This is because the formula is evaluating to True and it's therefore using a custom number format for display. This is the key difference between what we're doing here and what you were trying in the first post: we are NOT entering a value into the field, we are setting a formatting rule that affects how the cell is displayed. The former can never work conceptually.
Regrettably, whilst it OUGHT to work changing the formula to the current cell (because this formula is used for formatting, not for entering data into the cell), when I do so, it wont display the text. It will still do other formatting so if you want the cell to turn blue or something when it doesn't have a value, you can do that. But something must be preventing it displaying text. Sorry.
