It's strange because it has no traffic. I can navigate into this site but when i try to crawl it with Screaming Frog, i see a 503 error. How it's possible? How can i resolve this issue?
503 error doesn't necessarily have anything to do with traffic. It could be caused by anything really. Check your apache error log and see if anything shows up.
A lot of webservers have protective configurations to prevent them from being overloaded. When a crawling tool gets 503s, it's often a sign that either the rate of the crawl is too fast, or the number of simultaneous connections is too high. Check the tool for these settings and if they are high, reduce them to a level that does not generate any more errors.