air.cs.itu.edu.tr has an A (IPv4) record (160.75.154.23) but a request to this address over port 80 did not succeed. Your web server must have at least one working IPv4 or IPv6 address.
A timeout was experienced while communicating with air.cs.itu.edu.tr/160.75.154.23: Get "http://air.cs.itu.edu.tr/.well-known/acme-challenge/letsdebug-test": context deadline exceeded
Trace:
@0ms: Making a request to http://air.cs.itu.edu.tr/.well-known/acme-challenge/letsdebug-test (using initial IP 160.75.154.23)
@0ms: Dialing 160.75.154.23
@10000ms: Experienced error: context deadline exceeded
A test authorization for air.cs.itu.edu.tr to the Let's Encrypt staging service has revealed issues that may prevent any certificate for this domain being issued.
160.75.154.23: Fetching http://air.cs.itu.edu.tr/.well-known/acme-challenge/-OL54RerDpbBojrfJM4zqsYEp6pA0RpWTPKisLbcJcw: Timeout after connect (your server may be slow or overloaded)