"This subdomain is already taken." — the label is already registered under that platform domain. Try a different label, or a different platform domain.
"This label is reserved." or "This subdomain name is not available." — the label does not meet 20dragons' naming rules (see Registering a subdomain).
1–2 character labels are not available for ordinary registration; a 3-character label on a free-type domain requires a one-time credit charge.
My registration is pending
A "Pending review" status is normal if your registration requires review. See Registration review.
No action is normally required while it is pending.
Make sure the time on your device is correct — authenticator codes are time-based and depend on an accurate clock.
Make sure you are entering the current code, since codes expire quickly (typically every 30 seconds).
After 5 wrong attempts within 10 minutes, further attempts are temporarily blocked with "Too many two-factor attempts. Try again later." Wait and try again.
You can use a recovery code instead of an authenticator code if you have one saved.