Welcome to INTEL forum. Based on my understanding, Quartus Prime Pro Edition version 15.1 Beta is discontinued. We would suggest that you check with our distributor. Kindly contact your nearest Intel sales person or distributor https://www.intel.com/content/www/us/en/partner/where-to-buy/overview.html
Maybe the issue caused by corrupt or incomplete link file. Could you clear out your web browsers cache/temporary internet files and try download again it is recommended to download with combined file. https://www.intel.com/content/www/us/en/programmable/downloads/download-center.html
I am also having the same issue, I need to download Quartus 13.1 Web Edition. I have tried multiple browsers and computers but in all cases when I click accept on the legal disclaimer, simply nothing happens.
Previously I have used this site and there was no legal disclaimer, the download link worked properly then.