Export DataGrid Data to Excel Error: The given paths format is not supported

Im exporting datagrid data in excel and an unexpected error pop out because i want to export the data to excel by date and time

i have an error

An unhandled exception of type 'System.NotSupportedException' occurred in mscorlib.dll

Additional information: The given path's format is not supported.

Sub ExportToExcelGroupByGradeLevel()
    Dim dt As DataTable = New DataTable()
    For Each column As DataGridViewColumn In dgvExportData.Columns
        dt.Columns.Add(column.HeaderText, column.ValueType)
    Next
 
    For Each row As DataGridViewRow In dgvExportData.Rows
        dt.Rows.Add()
        For Each cell As DataGridViewCell In row.Cells
            dt.Rows(dt.Rows.Count - 1)(cell.ColumnIndex) = cell.Value.ToString()
        Next
    Next
 
    Dim result = From rows In dt.AsEnumerable()
                    Group rows By groupByKey = New With {Key .GradeLevel = rows("Grade Level")} Into Group
                    Select Group
    Dim folderPath As String = "C:\Excel\School Year\" & frmServerPanel.lblSchoolYear.Text & "\Enrolled\Reports\"
 
    If Not Directory.Exists(folderPath) Then
        Directory.CreateDirectory(folderPath)
    End If
 
    Using wb As XLWorkbook = New XLWorkbook()
        For Each item In result
            wb.Worksheets.Add(item.CopyToDataTable(), item.CopyToDataTable().Rows(0)("Grade Level").ToString())
        Next
        wb.SaveAs(folderPath & Convert.ToString(lbltime.Text & "S-DB-" & frmServerPanel.lblSchoolYear.Text & " " & "Group-By-Grade-Level.xlsx"))
        MessageBox.Show("Export Sucessfully.", "", MessageBoxButtons.OK, MessageBoxIcon.Information)
    End Using
End Sub
 
#End Region
 
Private Sub btnExport_Click(sender As Object, e As EventArgs) Handles btnExport.Click
    If cbFilterExport.Text = "Per Grade Level" Then
        ExportToExcelGroupByGradeLevel()
    End If
End Sub
 
Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick
    lbltime.Text = DateTime.Now.ToString("dd/MM/yyyy h:mm:ss tt")
End Sub

thank you for help well appreciated