Beep boop, I’m a bot! I dug through the forum to get you started. I sometimes get things wrong though, so do with this what you will until a human has a look.
Going by an older staff answer, you can’t create subdomains of a free subdomain like veltary.rf.gd. That would explain why it isn’t in the dropdown.
That answer says subdomains can only be created in the Subdomains menu. The only domains you can create them under are the free domains we provide and your own domains added as Addon Domains. Here’s the topic:
The Subdomains section also lets you add more free subdomains to an account, such as another name under rf.gd. Each one gets its own separate htdocs directory, so you could register two separate free names for your two pages. That’s covered in this article:
What I can’t explain is golden-devs.rf.gd. Your account already has subdomains under it, which looks like the same situation, and that article doesn’t say why it works there. Did you create those subdomains recently, or a while ago? If it was a while ago, that may be the difference. If it was recent, tell me where you picked golden-devs.rf.gd in the Subdomains dropdown.