Hi .. in your flow your injects are not set to Number 0 and Number 1
try this
[{"id":"e9110e0b.630f","type":"debug","z":"94ab22aa.035ba","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":930,"y":620,"wires":[]},{"id":"74db1897.77ce18","type":"inject","z":"94ab22aa.035ba","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"AirCon/SetOnOff","payload":"true","payloadType":"bool","x":240,"y":540,"wires":[["cc5d800d.3d5528"]]},{"id":"d4cda24.bd8556","type":"inject","z":"94ab22aa.035ba","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"AirCon/SetOnOff","payload":"false","payloadType":"bool","x":250,"y":600,"wires":[["cc5d800d.3d5528"]]},{"id":"bd8ef1c9.41c448","type":"modbus-flex-write","z":"94ab22aa.035ba","name":"","showStatusActivities":false,"showErrors":false,"server":"bf37180a.2d18e","emptyMsgOnFail":false,"keepMsgProperties":false,"x":690,"y":640,"wires":[["e9110e0b.630f"],[]]},{"id":"cc5d800d.3d5528","type":"function","z":"94ab22aa.035ba","name":"","func":"msg.payload = { \n \n value: msg.payload, \n 'fc': 5, \n 'unitid': 1, \n 'address': 0 , \n 'quantity': 1 \n \n} \n\nreturn msg\n","outputs":1,"noerr":0,"initialize":"","finalize":"","x":490,"y":640,"wires":[["bd8ef1c9.41c448"]]},{"id":"7e93a593.363944","type":"inject","z":"94ab22aa.035ba","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"AirCon/SetOnOff","payload":"1","payloadType":"num","x":230,"y":760,"wires":[["cc5d800d.3d5528"]]},{"id":"591ed24e.84118c","type":"inject","z":"94ab22aa.035ba","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"AirCon/SetOnOff","payload":"0","payloadType":"num","x":240,"y":820,"wires":[["cc5d800d.3d5528"]]},{"id":"e6cc9f4c.d8ea28","type":"comment","z":"94ab22aa.035ba","name":"or","info":"","x":230,"y":680,"wires":[]},{"id":"bf37180a.2d18e","type":"modbus-client","name":"","clienttype":"tcp","bufferCommands":true,"stateLogEnabled":false,"queueLogEnabled":false,"tcpHost":"192.168.1.66","tcpPort":"502","tcpType":"DEFAULT","serialPort":"/dev/ttyUSB","serialType":"RTU-BUFFERD","serialBaudrate":"9600","serialDatabits":"8","serialStopbits":"1","serialParity":"none","serialConnectionDelay":"100","unit_id":1,"commandDelay":20,"clientTimeout":1000,"reconnectOnTimeout":true,"reconnectTimeout":2000,"parallelUnitIdsAllowed":true}]
ps. modify the Modbus config to represent your settings.