www.phpmysqlcms.tk resolves OK but not phpmysqlcms.tk so make sure an A record or CNAME exists for phpmysqlcms.tk.

$ dig www.phpmysqlcms.tk +short
159.65.146.159

$ dig phpmysqlcms.tk

; <<>> DiG 9.10.3-P4-Ubuntu <<>> phpmysqlcms.tk
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 17705
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;phpmysqlcms.tk.            IN  A

;; AUTHORITY SECTION:
phpmysqlcms.tk.     1796    IN  SOA ns1.digitalocean.com. hostmaster.phpmysqlcms.tk. 1543820849 10800 3600 604800 1800

;; Query time: 28 msec
;; SERVER: 127.0.1.1#53(127.0.1.1)
;; WHEN: Tue Dec 04 13:30:03 CST 2018
;; MSG SIZE  rcvd: 110

Hence the error: “The server could not resolve a domain name”

Cheers

@tkhara

The domain phpmysqlcms.tk is not defined you need to associate an IP address with the domain name so you can create a HTTPS certificate for it.