This happens when you access the download link from a different browser than the one in which you are logged in to your form builder account on 123FormBuilder.com.
For example, let’s say that you normally use browser A when working with your forms. But browser B is set as default for opening email links. The download link will not work, because it will open up in browser B, whilst you are authenticated on your 123FormBuilder account in browser A.
Therefore, make sure that you are logged in to your 123FormBuilder account in the same browser in which the download link opens.