I am also not able to add the domain with ipv4, please refer below link.
https://prnt.sc/qnedsh
root@linux:# dig a ipv6.1dollarvpspro.com
; <<>> DiG 9.11.3-1-Debian <<>> a ipv6.1dollarvpspro.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 31929
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;ipv6.1dollarvpspro.com. IN A
;; ANSWER SECTION:
ipv6.1dollarvpspro.com. 14399 IN A 137.59.55.249
;; Query time: 70 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Tue Jan 14 13:36:25 IST 2020
;; MSG SIZE rcvd: 67