The root of a domain (e.g.
example.com. without any subdomain) contains important DNS records like the SOA (start of authority) record that is specific to this exact domain:
$ dig example.com SOA
;; ANSWER SECTION:
example.com. 3230 IN SOA sns.dns.icann.org. noc.dns.icann.org. 2015082680 7200 3600 1209600 3600
A CNAME record (like the one you have created for
www.cloudstorm.io) works as an alias: all other records for this subdomain are ignored, and the records from the destination (
dns.strikingly.com in your case) are used instead.
For this reason, a CNAME record cannot be used at the root of a domain: It would instruct the DNS client to use the SOA record of a different domain, which violates the DNS specification.
So if you can't use a CNAME record at the domain root, what can you use?
You will have to configure all necessary records manually (A for IPv4 addresses, AAAA for IPv6 addresses, MX for mailserver lookups, etc.).
In your use case (with a website hosted by a third-party provider) that will lead to problems whenever that provider decides to make any changes to their network: Unlike with a CNAME, you've copy-pasted their IP addresses into your own DNS zone, so they won't be updated automatically.
You can either take that risk or, preferably, contact Strikingly and ask whether they will host and maintain the entire DNS zone for your domain (thus also updating the records in the domain root whenever it becomes necessary).
Strikingly also suggests a workaround for when you don't have sufficient options to modify your DNS zone, but their support might have a less hackish solution for you.