LGT92 not joining lora

I have a question, i have the best configuration but i want to change the frequency, but when i put the AT+RX2FQ=915000000 only says AT_BUSY_ERROR, any idea? @brocaar

I already have it, but thanks anyway, in the pic below i need to implement the command

but always when i try to execute it as: AT+RX2FQ=915000000 the answer always is AT_BUSY_ERROR (pic below)

LoRaWan US915 defines 64 channels starting from 902.3Mhz (each 125KHz large), separated by 200KHz each.

Channel Frequency
0 902.300.000 Hz
1 902.500.000 Hz
63 914.900.000 Hz

there are no this frequency in the US915

Thanks brother for ur help.