VBA Macro for Removing #N/As

Hi all,

I just registered to the forum so nice to meet you all. Before we start, a little bit of my background so you can know who you're helping. I'm a 24yo french finance master's degree student. I'm currently doing my master's thesis whose subject is mutual funds' performance and fees. I basically want to know if an investor who pays more fees will receive more performance, as fund managers claim fees such as management fees are the reflect of their skills.
So to be able to show whether fees and performance are positively correlated or not, I downloaded a massive database with 10k funds and their monthly NAV (or price) starting from January 2004 (so roughly 200 observations/fund). Now of course, I have lots of #N/A in my database and for 2 reasons :
- market was closed that day
- or the fund was not alive yet
I would like to clean that database and get rid off these #N/As. To do so, I want to target each #N/As that are comprised between 2 values and use the average of these 2 values to replace the #N/A.

I just wrote a VBA code to help me do that quickly but it doesn't work :

Public Sub RemoveNA()

Dim row_number As Long
Dim col_number As Long
Dim line_number As Long
Dim column_number As Long

column_number = InputBox("How many columns?")
line_number = InputBox("How many rows?")

For col_number = 1 To column_number
    For row_number = 1 To line_number
    If Cells(row_number, col_number).Value = "#N/A" Then
        Cells(row_number, col_number).Value = 0
    End If
    Next row_number
Next col_number

End Sub

Here is a snapshot of what my excel look like :

LP61500731 LP61501237 LP61500989 LP61501059 LP40209239 LP40209256
Net Asset Value 31/08/2020 51.44 30.01 18.30 13.43 11.78 14.13
Net Asset Value 31/07/2020 52.48 30.65 18.63 13.71 10.94 13.45
Net Asset Value 30/06/2020 52.77 30.75 18.70 13.64 10.42 12.84
Net Asset Value 29/05/2020 52.96 30.79 18.74 13.39 10.06 12.65
Net Asset Value 30/04/2020 51.78 29.99 #N/A 12.95 9.57 12.07
Net Asset Value 31/03/2020 44.53 25.96 15.74 11.67 8.52 11.13
Net Asset Value 28/02/2020 52.21 30.00 18.38 13.18 #N/A 12.60
Net Asset Value 31/01/2020 58.71 33.51 20.60 14.06 12.48 13.68
Net Asset Value 30/12/2019 61.57 34.98 21.52 15.20 13.07 13.63

Hope someone understands my problem and can give me any clue about how to solve it

Have a wonderful day,

Alexis