unable to boot from SDCard

[Charles Tarkowski]

have installed eMMC 32 GB and tried with Debian, all OK, start, SSH via Putty, nand-sata-install, boot without SDCard, all fine.
then i understand, that it is 64-Bit and wine32 do not function in this enviroment, what i need, without this = dead Piece of Hardware for me.
So finding Ubuntu-Version (Bionic with 32 Bit)  github.com_bionic-minimal-rockpro64-0.8.3-1141-armhf.img, tried it and
Hurra, it works, with wine32, x2go, xfce, all I want...(alll on SDCard)
Now beeing unfearful, and try to transfer, or make it new with the eMMC, put the image on /tmp and use
dd Cmd, and now all is gone,
i think i had to delete the Debian-System on eMMC, i do not do it,

Now i try to start without eMMC, and some trial of SDCards, with Jumper (north of the intern button)
all combinations, but no more boot is possible,
the LED's at Network-cable are out,

i do not understand the Connection via TTY and serial Connector, i do not have USB Hardware for this,
some other older PC with seriell Connector is perhaps avaiible.

Pleas help me, any suggestions are welcome..
Charles