I have already registered on netacad and it's just that when I open packet tracer the user login page does not show up, it's just blank.
Try using an Internet connection for initial registration
I had the same issue when I tried using Packet Tracer for the first time from my company's intranet. I brought my laptop home and was able to successfully log in from my Internet connection. Now that I've logged in, I'm able to use Packet Tracer along with my company's VPN. So it looks like there's a one-time login step that won't work through (at least my company's) intranet. Note that while on the intranet I did try setting the proxy settings and that didn't solve the issue.