Most often the problem is related to the fact that site builder which you use is running under HTTP protocol whereas DirectAdmin is running under HTTPS protocol. Since site builder in DirectAdmin Evolution skin is tried to be open in an iframe it is blocked due to browser CORS policy and the progress is stuck on loading step.
To fix this issue you can either enable HTTPS protocol for site builder (see this page for more details on how to do this: https://site.pro/esc/FAQ-Para-Resellers/#kb516) or update Site.pro plugin on DirectAdmin by this instruction.