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Certain top-level domains are locked for a period after they have been registered, transferred, or recently renewed. The lock lasts 60 days, during which the domain name cannot be transferred.
This also applies if you change the ownership of a domain, though in that case there is an option to choose whether the lock should be activated or not.
Note that this only applies to generic top-level domains (gTLDs) such as .com, .net or .org, and not country-code domains such as .se or .no.