Can you kindly write to selfstudysupport@pega.com? Without more information like the user id you use to log into Pega Academy, the Course and exercise you are working on, it's really difficult to help you by just simply saying you cannot access the system!

i just sent email to the selfstudy 

Thanks. I triaged your case and the problem you cannot log in is because you have deleted the Application rule. This rule is required for your LeadArchitect account to login, that's why when it's missing, the system does not log you in.