"Initial setup" not possible with Matek 743

I try to set up a Matek 743 with the latest firmware and APM. I followed exactly the documentation and used the STM32 tool to install arduplane_with_bl.hex form MatekM743 firmware folder. This step finished (two times) without any problem on a Ubuntu PC. When I connect with APM to the Matek743, I can configure the the Matek (CONFIG/TUNING tab) but I cannot do the “Initial Setup” step. When I hit “Install Firmware”, I always get the message “no valid device found”.
I hesitate to continue, because I’m not sure if it really works. Although it seems to be possible to configure the Matek board.
thanks for any help, regards, Gerd

I have to add a correction: in fact it is possible to connect and to do the Initial Setup. But it is not possible to “Install Firmware”. When I “disconnect” and then hit “Install Firmware”, I always get the message “no valid device found”.
Do I have to press the boot button to install firmware with APM? Not sure about this point. But I tried either way without success.