Hi

Dim the variables properly. This should work.

Code:
Sub SearchBookss()
    
    Dim Sht             As Worksheet
    Dim SearchWord      As String
    Dim WordAddress     As Range
    Dim CheckCell       As String
    Dim Addr            As String
    Dim i   As Long
    
    Application.ScreenUpdating = 0
    
    SearchWord = InputBox("Enter the string to search for")
    On Error GoTo Xit
    For i = 1 To Workbooks.Count
        For Each Sht In Workbooks(i).Worksheets
            Application.Goto Sht.Cells(1)
FindAnother:
            Set WordAddress = Sht.UsedRange.Find(What:=SearchWord, lookat:=xlPart, MatchCase:=False)
            If Not WordAddress Is Nothing Then
                CheckCell = WordAddress.Address
                Do
                    Set WordAddress = Sht.UsedRange.FindNext(WordAddress)
                    Addr = WordAddress.Address
                    MsgBox Workbooks(i).Name & Chr(13) & Sht.Name & vbLf & Addr
                Loop Until Addr = CheckCell
                Application.Goto Sht.Range(CheckCell)
            End If
        Next
NextBook:
    Next i
    Application.ScreenUpdating = 1
    Exit Sub
Xit:
    MsgBox Err.Description
    Application.ScreenUpdating = 1
End Sub