Unlock cell/s dependant upon entry in another cell

Here's an example.

Open a new workbook, select A1:A10, and unlock the cells.

Then protect the workbook, allowing selection of unlocked cells only.

Then right-click the Sheet tab, select View Code, and paste this in the window that opens:

Private Sub Worksheet_Change(ByVal Target As Range)
    Me.Protect Password:="", UserInterfaceOnly:=True
    With Target
        If .Count = 1 And .Column < Me.Columns.Count Then .Offset(, 1).Locked = False
    End With
End Sub

Close all that and go back to Excel.

You can only select A1:A10. Entering something in one of those cells will unlock the cell to the right; enter something in that cell, and it will unlock the next.

If you've never used VBA, this might not be the easiest place to start.