Hi Alireza,
I'm not sure if understand your problem correctly: you are not able to write to the MUX register after a power up, correct?
After a power up, the ADS1256 will reset and be in RDATAC mode. If you you want to change the register settings after power up, I recommend sending a SDATAC command first thing before writing to the registers. While the device is in RDATAC mode it is only expecting a SDATAC or RESET command.
FYI: (page 35 of the data sheet has some typos and calls the "SDATAC" command "STOPC").
Best regards,
Chris