Server DNS address could not be found. While ping and traceroute are working correct!

Hi, yesterday I created domain records, but it still shows me “Server DNS address could not be found.”. My whois aecs.pl:

DOMAIN NAME:           aecs.pl
registrant type:       individual
nameservers:           ns1.digitalocean.com.
                       ns2.digitalocean.com.
                       ns3.digitalocean.com.
created:               2017.09.15 10:47:42
last modified:         2017.09.15 12:41:55
renewal date:          2018.09.15 10:47:42

no option

dnssec:                Unsigned


REGISTRAR:
home.pl S.A.
ul. Zbożowa 4
70-653 Szczecin
Polska/Poland
+48.914325555
+48.504502500
https://home.pl/kontakt

WHOIS database responses: http://www.dns.pl/english/opiskomunikatow_en.html

WHOIS displays data with a delay not exceeding 15 minutes in relation to the .pl Registry system
Registrant data available at http://dns.pl/cgi-bin/en_whois.pl

my traceroute aecs.pl:

traceroute to aecs.pl (165.227.160.57), 64 hops max, 52 byte packets
 1  funbox (192.168.1.1)  1.866 ms  1.788 ms  1.497 ms
 2  wro-bng2.tpnet.pl (80.50.18.74)  3.220 ms  3.539 ms  3.578 ms
 3  wro-r1.tpnet.pl (80.50.18.73)  3.757 ms  2.725 ms  2.830 ms
 4  war-r1.tpnet.pl (194.204.175.113)  13.738 ms  19.738 ms  13.848 ms
 5  war-b2-link.telia.net (213.248.89.85)  29.031 ms  28.103 ms  29.051 ms
 6  ffm-bb3-link.telia.net (80.91.246.174)  29.209 ms
    ffm-bb3-link.telia.net (213.155.133.90)  28.576 ms
    ffm-bb3-link.telia.net (80.91.245.250)  28.719 ms
 7  ffm-b4-link.telia.net (62.115.120.2)  29.096 ms
    ffm-b4-link.telia.net (62.115.120.6)  29.814 ms
    ffm-b4-link.telia.net (62.115.120.2)  28.317 ms
 8  digitalocean-ic-310409-ffm-b2.c.telia.net (213.248.103.26)  28.678 ms
    digitalocean-ic-310410-ffm-b2.c.telia.net (213.248.75.42)  28.989 ms  29.169 ms
 9  * * *
10  165.227.160.57 (165.227.160.57)  30.458 ms  29.385 ms  29.461 ms

everything seems to be ok, why does this error shows?