The serial don't send me the same data with the same string

Using Arduino Programming Questions
3

Hello, do yourself a favour and please read How to get the best out of this forum and modify your post accordingly (including code tags and necessary documentation of your ask like how things are wired and powered for example).

you probably should look into byte arrays like

const byte Chaine_Intro[] = {0x7E, 0x01};
const byte Adresse[] = {0xFE, 0xFE};
const byte Cmd_INST[] = {0x49, 0x4E, 0x53, 0x54};
const byte Msg_Sep[] = {0x00};
...