How do I add a value of one cell to another cell when criteria is met

Option Explicit

Private Sub Worksheet_Change(ByVal Target As Range)
    
    'This checks for any entry in column B
    If Application.Intersect(Range("B:B"), Range(Target.Address)) Is Nothing Then
        Exit Sub
    End If
    
    'Or check that it is the specific range you are interested in
    If Target.Address <> "$B$24" Then Exit Sub

    'If there is nothing currently in D24 then nothing more to do
    If IsEmpty(Range("D24")) Then Exit Sub  'Note you cannot use IsBlank

    'If user did not enter a number then there is nothing more to do
    If Not IsNumeric(Target.Value) Then Exit Sub

    'If the current value in D24 is not numeric then nothing more to do
    '(Never trust a user to enter the data you are expecting!)
    If Not IsNumeric(Range("D24").Value) Then Exit Sub
    
    'Add the new value to the running total
    Range("D24").Value = Range("D24").Value + Target.Value

End Sub