Let's Debug

Test result for www.internal.cz using http-01

A fatal issue occurred during the DNS lookup process for www.internal.cz/A.
DNS response for www.internal.cz/A did not have an acceptable response code: SERVFAIL
No valid A or AAAA records could be ultimately resolved for www.internal.cz. This means that Let's Encrypt would not be able to to connect to your domain to perform HTTP validation, since it would not know where to connect to.
No A or AAAA records found.

Submitted May 4 23:18:45 2020. Sat in queue for 2ms. Completed in 7s. Show verbose information.