Why can I not change the digital inputs polarity?

The digital inputs polarity register shall be read in binary notation: each bit corresponds to the polarity of each of the digital inputs. This should be writable whenever the drive is not in the switch ON state, meaning it has to be modified before enabling the power stage.

Links to register:

0x604 - Digital inputs polarity - SPI communication drives

0x2604 - Digital inputs polarity - EtherCAT/CANopen drives