Ok, so your Modbus device doesn’t support the function code that allows individual bits to be written to.
You’ll have to use some kind of workaround where you read, mask, and write the whole register back.
One approach using a derived tag is discussed here: Troubles making a Derived Boolean Tag work as a mask for a Short Tag - #2 by Kevin.Herron
Another that uses scripting here: Modbus UA - #5 by Travis.Cox