Cannot read flash with bossac program

Hello list,
I try to read the content of my nano 33 ble. I use the bossac program from

.arduino15/packages/arduino/tools/bossac/1.9.1-arduino1/bossac

But:

$ bossac -r out.bin
Read 1048576 bytes from flash
[                              ] 0% (0/256 pages)
SAM-BA operation failed

While:

$  bossac -e -w -v -b ble2.arduino.mbed.nano33ble.bin 
Erase flash

Done in 0.002 seconds
Write 292720 bytes to flash (72 pages)
[==============================] 100% (72/72 pages)
Done in 11.231 seconds
Verify 292720 bytes of flash
[==============================] 100% (72/72 pages)

Succeeds.

What's up?

Did you put it in bootloader mode by pressing the reset button twice so that the "L" led starts fading in and out?

Yes.

it is not a SAM MCU. I think Arduino made a bootloader only to upload to this nRF MCU,

Thanks for the reply.
Yes, but I am using the adapted version that is for the nano 33 ble.
Note that I am using it regularly for the last month without problems.
I only recently noticed that the read option did not work, hence my question.