Display Domain Name instead of IP for bucket content in google cloud platform?
The name of the bucket (as you create it) can be in a form of a domain name (domain ownership verification will be required).
If the bucket is to be accessed via LB+CDN - skip the above, you then create a load balancer and then point your dns A record to the LB's ip (in case of https you also configure SSL for this domain by either uploading your certificate or opting to use Google Managed one).
Regarding the error you get - is it possible you already have an A record that you are trying to add?