Hi kraust,
Please make sure the following things correctly.
1) Did you copy all the files (boot.imx, uImage and dtb) using sudo. And Did you copy the u-boot image as documented in the timesys document (https://linuxlink.timesys.com/docs/gsg/twr_vf600).
ex:sudo dd if=u-boot.imx of=/dev/sdX bs=512 seek=2
sd X* can be obtained by running dmesg in the terminal.
2) After placing the sdcard in to the TWR-VF65GS10 bord did you make sure that which serial port TWR-VF65GS10 is connected?
Again this can be obtained running dmesg in the terminal. Once after that you need to configure the minicom using the same serial port you got (Ex: ttyUSB0)
If you have done the above mentioned steps correctly then resetting the board take you to the Boot menu. Once after giving the boot args you will be able to boot the board. Please also try and format the SD card as per timesys getting started guide. If issue persists please share the error you are getting and U-boot version.
Thanks
Timesys support.