Allow data Input but don't allow Edit

Try to use Data Validation in combination with Worksheet Protection.

Insert a New VBA Code:

In the VBA Editor, double-click on the sheet where you want this behavior (e.g., "Sheet1")

Private Sub Worksheet_Change(ByVal Target As Range)
    ' Check if the cell is within the allowed range for input
    If Not Intersect(Target, Me.Range("A1:A100")) Is Nothing Then
        ' Disable editing after the first input
        If Target.Value <> "" Then
            Target.Locked = True
        End If
    End If
    ' Protect the sheet to prevent further editing
    Me.Protect Password:="yourpassword"
End Sub

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    ' Unprotect the sheet to allow initial editing
    Me.Unprotect Password:="yourpassword"
End Sub
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Remember to replace "A1:A100" with the range of cells where data entry is allowed, and also replace "yourpassword" with a password of your choice.

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2nd Method:

Select all the cells in the sheet.
Right-click and choose Format Cells.
Go to the Protection tab and uncheck the Locked option.
Select all the cells in the sheet.
Right-click and choose Format Cells.
Go to the Protection tab and uncheck the Locked option.
(This will make all cells initially editable)

Select the range of cells where you want to allow data input (e.g., A1).
Right-click and choose Format Cells.
Go to the Protection tab and check the Locked option for these cells.

Go to the Review tab and click Protect Sheet.
Set a password to protect the sheet.
Please ensure Select unlocked cells is the only option checked.

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3rd Method:

If you want to keep it simple and just give users a warning when they attempt to overwrite existing data:

Select the range where users are inputting data (e.g., A1)
Go to the Data tab > Data Validation > Data Validation
Set the Allow option to Custom
Enter this formula: =ISBLANK(A1) (replace A1 with the first cell in your range)
Set an Input Message and Error Alert to notify users that overwriting data is not allowed

Use conditional formatting to visually indicate which cells are filled.
Then select the range (e.g., A1)
Go to Home > Conditional Formatting > New Rule > Use a formula to determine which cells to format.
Use this formula: =NOT(ISBLANK(A1))
Set a fill color to indicate that the cell is already filled

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NOTE: in the above VBA Method is the most effective, as it will completely prevent further editing after the initial input, worksheet Protection can also be used to lock cells after initial input and data Validation can be used for a softer approach to give users a warning instead of completely blocking editing.