We can create a warning message with custom text and ISBLANK function to indicate there are blank cells in a specific range in Excel. And you can do as following:

Step 1: In a blank cell, enter the formula ="There are still "&COUNTBLANK(B1:B16)&" questions you have not answered!"; and press the Enter key.

Note: In the formula above, the B1:B16 is the specific range of Answer column where you want to count the blank cells, and you can change it as you need.

Step 2: Then format the cell to make it outstanding. Select the cell, and click the Home > Font Color > Red, next click the Bold button, and then specify the font size as you need.

Quickly enter dash or certain text of NA into all blank cells of selected range in Excel

Kutools for Excel's Fill Blank Cells utility can help you quickly enter a certain text, such as "Warning" into all blank cells in the selected range only with several clicks in Excel.

We can also add a help column to display warning messages besides each blank cell in specific range. And you can do as following:

Step 1: in the Cell C2 besides the Answer column, enter the formula =IF(ISBLANK(B2),"Please Answer This Question",""), and drag the Fill Handle to the range you need.

Step 2: Next select the range you apply the formula, and then format with clicking Home > Font Color > Red and clicking the Bold button.
So far, we have added the warning messages behind each blank cell of Answer column. See the screen shot below:

We can also apply a VBA macro to check if there are blank cells in the specific range in Excel. And you can do it as following:

Step 1: Press the Alt + F11 keys to open the Microsoft Visual Basic for Application window.

Step 2: Click the Insert > Module, and then paste the following VBA code into it.

VBA: Warn if blank cells exist in specific range

Sub BlackCell()
Dim Rng As Range
Dim WorkRng As Range
On Error Resume Next
xTitleId = "Kutools for Excel"
Set WorkRng = Application.Selection
Set WorkRng = Application.InputBox("Range", xTitleId, WorkRng.Address, Type:=8)
Set WorkRng = WorkRng.SpecialCells(xlCellTypeBlanks)
If Err = 0 Then
MsgBox "You don't answer all of questions!"
End If
End Sub

Step 4: Now the Kutools for Excel dialog box comes out, and please specify the range you want to check whether blank cells exist or not, and click OK button to close the dialog box.

If blank cells exist in the specific range, it will pop up a dialog box as the following screen shot shown:

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