Reasons a domain name cannot be registered:
1. It is not yet available for registration.
2. It is in Sunrise, a phase that is only open to mark holders with a valid SMD file from the Trademark Clearinghouse. Read more at the TMCH website.
3. It is DPML blocked from registration by a trademark holder. You can verify this by going searching WHOIS and typing in the name with dpml.zone or plus.dpml.zone appended (example.dpml.zone, or (example.plus.dpml.zone).
4. It is registered by someone else. You can verify this by checking WHOIS.
5. It is reserved and not available for registration. There are several reasons a name might be reserved:
b. ICANN mandated temporary Name collision mitigation (NXD); an example list is here.
c. Reserved for Donuts use. Again detailed in Specification 5 of the Registry Agreement.
Please check these sources and work with your Registrar to find a name that is available for your use.