This occurs when a firewall or WAF blocks all connections from Acunetix. You probably can still connect to the website from your browser, since the firewall is not blocking your IP address. You will need to confirm that your firewall is not blocking the connection from Acunetix. All Acunetix Online scans are done from scanners.acunetix.com.
If the target is hosting a web site, you can check the connection from Acunetix by re-verifying the scan target. This will prompt Acunetix to request the verification file, and report back any errors encountered.