It could also mean that it didn't find a match and cannot .Activate the cell.

Try something like this...

    Dim Found As Range, x As Long, u As Long
    
    x = 2
    
    Set Found = Sheets("Dataset2").Range("AR:AR").find(What:=Sheets("Dataset1").Cells(x, 1).Value, _
                                                       LookIn:=xlValues, _
                                                       LookAt:=xlWhole, _
                                                       SearchOrder:=xlByRows, _
                                                       SearchDirection:=xlNext, _
                                                       MatchCase:=False)
    
    If Not Found Is Nothing Then
        MsgBox "Match found in row " & Found.Row, , "Match Found"
        u = Found.Row
    Else
        MsgBox "No match found for '" & Sheets("Dataset1").Cells(x, 1).Value & "' ", , "No Match"
    End If