Thank you @thedaveCA for your response.
I corrected allika.in nameservers to Cloudflare only and the issue is resolved.
However, I already removed he.net nameservers and updated Cloudflare nameservers at datascience.eu.org registrar two days ago. This is a view from my registrar:
Configuration for datascience.eu.org
Current DNS records
NS ANASTASIA.NS.CLOUDFLARE.COM.
NS LARS.NS.CLOUDFLARE.COM.
History for datascience.eu.org
#### From Sept. 1, 2020, 2 p.m. to ...
NS ANASTASIA.NS.CLOUDFLARE.COM.
NS LARS.NS.CLOUDFLARE.COM.
#### From Sept. 1, 2020, 10:49 a.m. to Sept. 1, 2020, 2 p.m.
NS ANASTASIA.NS.CLOUDFLARE.COM.
NS LARS.NS.CLOUDFLARE.COM.
#### From Jan. 18, 2020, 1:46 p.m. to Sept. 1, 2020, 10:49 a.m.
NS NS1.HE.NET.
NS NS2.HE.NET.
NS NS3.HE.NET.
NS NS4.HE.NET.
NS NS5.HE.NET.
I am unable to understand where’s the issue.