With this bit of code it works.
[{"id":"4199525f95a962f2","type":"inject","z":"65c9b63cb09879a0","name":"","props":[{"p":"payload.data_day.temperature_max","v":"[1,2,3,4]","vt":"json"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","x":2840,"y":1530,"wires":[["b0ef4d2772abb305"]]},{"id":"b0ef4d2772abb305","type":"function","z":"65c9b63cb09879a0","name":"","func":"msg.topic = \"ECRAN-ENTREE/cmd\";\nmsg.payload = Math.round(msg.payload.data_day.temperature_max[1])\nmsg.payload = msg.payload + \"°C\" ;\nmsg.payload = `NEXTION,home.t1.txt=\"${msg.payload}\"`;\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":3000,"y":1530,"wires":[["a99e797e0140c213"]]},{"id":"a99e797e0140c213","type":"debug","z":"65c9b63cb09879a0","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":3170,"y":1530,"wires":[]}]
But I don't know what your real incoming message looks like.
How about you post what the incoming message is too?