i wonder if that is legal or not to ask purchase code in the installation process to complete the installation of a script sold on code canyon ?
No, it’s not allowed, but there’s a lot of authors that do it regardless.
In my application’s installer I included a “Sign in with Envato” page where they can verify their purchase and enable automatic updates. But it has an option to skip it. 
According to the email reply from evanto right management team it seem to be legit to do it.
Hey Guys,
We also did not find any article telling us that protecting a product with a valid Purchase code is against Envato’s policy.
We even made the Envato Purchase code and Support Checker wich ityself uses a purcgase code verify step
All the best and good luck with sales!