Hi, this is not an issue for accessing a bit because the bit is extracted after reading the register.
The message you get means that the controller responded with an error message for that Register (regardless of bit). Please verify you are using the right address mapping. I recall that on Wago controllers actual modbus addresses start at 40000, but I am not copletely sure. Also I believe there is a different mapping for reads and writes, though writable addresses are also readable. Possibly you are accessing a register that is beyond the memory of the controller.
I hope this helps to narrow the issue. You may also look at the example files provided by Wago you will find in our site.