I was also getting the "not in bootloader mode" error as well - and with a very trusted USB-C to USB-C cable! Then remembered I had the FlipperZero box with me and tried that cable - which WORKED! So, this is not the main reason to buy one obviously, but if you do have one and you're experiencing this error on a Mac (M1) then this is something to try. The "not in bootloader mode" error is a little misleading in this case as the cardputer was doing its part, but the first "trusted" cable was not!