When Teensy is running a normal program (not in bootloader mode) or when no Teensy is connected at all, you can't load new programs. These buttons are disabled under those conditions. They enable only when a Teensy is detected running in bootloader mode.
First of all, thank you for your response.
Can the bootloader mode also be activated from within the software, so that the prog button no longer needs to be pressed?
Thanks in advance.