What may be a reason for "Domain name not available to access." error?

I installed Domain Mapping plugin on multisite with sites installed in folders (not in subdomains). Copied sunrise.php, modified config.php, added IP in Domain Mapping settings.

Problem is, when I add domain in site's dashboard I get error "Domain name not available to access". Domain has the A record properly configured - https://www.whatsmydns.net/#A/internetowymarketingsieciowy.pl

What may be the cause of this error?