When I run this command :'./nym-mixnode run --id zzznym', an error occurs

[eric-xdp]

failed to register with the validator - ValidatorError("mixnet is already at capacity")

[jstuczyn]

Hi, it looks like you're trying to join the network, which as the error tells you, is currently at its capacity. Your best bet would be to watch the network and wait for somebody else to drop out so that a spot opens up. Alternatively you might want to wait for the next release when the limit should get changed.