Hi Robespierre,
 
I understand how confusing this can be. Diagnostic tests can be completed as the first activity when you first start practicing in any of the Test Prep areas (SAT, LSAT, and Praxis). However, these diagnostics can be skipped if you head directly toward the practice content.

If you have already completed an activity, the diagnostic tests disappeared because of that and it's not possible to get them back unless you create a new account. But don't worry, your future recommendations will always be based on your current performance, so you will always get recommended skills for your current level. 

Best regards,