Attempt to create a pointer to a constant

I get this Error every time and i have no idea why thats happening.

char buffer_RS232_BLE[buffer_size_RS232_BLE];
...
    **if (strstr(buffer_RS232_BLE, "timer") !=NULL)**
    {
        fprintf(BLE,stunden+"H"+minuten+"M"+sekunde+"S");
        next_in_RS232_BLE = 0;
    }