Writing to add that I solved this issue by mapping the toggle to new memory tag and then using a value changed script to send the value to the DI tag.
Apparently the root of the issue was that the coils can only be written to in Modbus RTU. The switch was trying to read the value.
I anticipate any future programming will have to be mapped to the memory tag to ensure the value of the memory tag/toggle matches the position of the valve.
def valueChanged(tag, tagPath, previousValue, currentValue, initialChange, missedEvents):
tag = "DI TAG HERE"
value = system.tag.readBlocking(MEMORY TAG HERE)
system.tag.writeBlocking(tag, value)