To extend on @kersing’s response a bit: indeed, gateways must adhere to a country’s legal maximum duty cycle. (And such maximum duty cycle is not some daily maximum, but applies at all times, 24/7, and is only related to the last transmission time.) But gateways are not affected by the 24 hours TTN’s Fair Access Policy. The latter only applies to devices, not to gateways.
True, as (current) gateways are half-duplex and cannot receive while transmitting, a network could also limit the number of downlinks that a gateway can transmit even though duty cycle regulations would still allow for it. But as far as I know, TTN has not implemented that. Also, a better solution would be to deploy more gateways, and to enforce the Fair Access Policy for devices (which is currently not enforced).
You’ll never get an error boldly saying “Keys does not match”. Instead, an RN2xx3 might suggest to make sure the keys (and coverage) are okay, but it will never know what’s the actual problem. To understand no_free_ch see Duty Cycle in the documentation.