Solution - type XLWorkbook is not defined
Using wb As XLWorkbook = New XLWorkbook()
wb.Worksheets.Add(dt)
wb.Worksheets.Add(GridView1)
wb.Worksheets.Add(GridView2)
Response.Clear()
Response.Buffer = True
Response.Charset = ""
Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
Response.AddHeader("content-disposition", "attachment;filename=Medications_Report_" & DateTime.Now.ToShortDateString().Replace("/", "_").Replace(" ", "_") & ".xls")
Using memoryStream As MemoryStream = New MemoryStream()
wb.SaveAs(memoryStream)
Dim bytes As Byte() = memoryStream.ToArray()
memoryStream.WriteTo(Response.OutputStream)
memoryStream.Close()
Response.Flush()
Response.[End]()
End Using
End Using