I am trying to register a domain that appears as expired. Why can’t I register it?

Domains have a life cycle that depends on each registry.

Generally, when a domain expires, it passes to "RenewGrace", then to "RedemptionPeriod" and finally to "Pending Delete", although this is not the case in 100% of the cases.

You can check the removal process and the exact time for each extension here

Once this life cycle ends, the domain becomes free again for registration.