Complile Error: Type Mismatch ??? After adding error trap

So this was my code and it worked just fine.

Private Sub ShowOnlyRowsThatAreTrue()   'This shows only the rows that are to be included
'in the systems included section
    Application.ScreenUpdating = False
    Dim FirstSystems_Row As Long
    Dim n As Integer
    Dim i As Integer
    
    FirstSystems_Row = Cells.Find("Systems included in this proposal are as follows:", , xlValues, xlWhole).Offset(2).Row
        For i = 5 To 23
            If Sheet1.OLEObjects("CheckBox" & i).Object.Value = True Then
                Sheet5.Rows(FirstSystems_Row).EntireRow.Hidden = False
            End If
            FirstSystems_Row = FirstSystems_Row + 1
        Next i
Application.ScreenUpdating = True
End Sub

I wanted to error trap for when the find command doesnt find anything - so I changed my code to this and now it doesnt work. I get an COMPLILE ERROR: TYPE MISMATCH. What does that mean?

Private Sub ShowOnlyRowsThatAreTrue()   'This shows only the rows that are to be included
'in the systems included section
    Application.ScreenUpdating = False
    Dim FirstSystems_Row As Long
    Dim n As Integer
    Dim i As Integer
    
    FirstSystems_Row = Cells.Find("Systems included in this proposal are as follows:", , xlValues, xlWhole).Offset(2).Row
    If Not FirstSystems_Row Is Nothing Then
        For i = 5 To 23
            If Sheet1.OLEObjects("CheckBox" & i).Object.Value = True Then
                Sheet5.Rows(FirstSystems_Row).EntireRow.Hidden = False
            End If
            FirstSystems_Row = FirstSystems_Row + 1
        Next i
    Else
       Msgbox "ERROR! Systems included in this proposal are as follows: Text is not found on the cover letter."
       Application.ScreenUpdating = True
    End If
End Sub

All I changed was:

If Not FirstSystems_Row Is Nothing Then
'code
Else
   Msgbox "ERROR! Systems included in this proposal are as follows: Text is not found on the cover letter."

End If