Try these steps: clear browsing data and cookies, disable unnecessary extensions, ensure the download is from the official PokerStars site, and try a different browser. If none of these work, contacting PokerStars support for specific download troubleshooting might be necessary.
Make sure you have the latest version of the PokerStars app installed and restart it. If you have a firewall or security software, make sure it allows the PokerStars app to access the Internet. Restart your PC / smartphone.