Hello @pengyong_zhang
I even tried that but it didn't work. So, after all of this I downloaded Linux and followed the process below:
I downloaded the following packages and binaries to create the standalone image(flash.bin): uboot-imx.git, imx-atf.git, firmware-sentinel-0.10.bin and firmware-upower-1.3.0.bin. I have extracted all the required files from the mentioned packages to create the flash.bin. I used VSCode and imported evk9mimx8ulp_hello_world_cm33 project. I renamed the hello_world.bin to m33_image.bin. Then used imx-mkimage with the following command to generate the flash.bin "make SOC=iMX8ULP REV=A2 flash_singleboot_m33". Then used "uuu -b emmc flash.bin" but after this ı get HID(W):LIBUSB_ERROR_TIMEOUT. In the serial port it prints:
"invalid container image
Download -65712, Total size 0
ROM download failure -65712
Wrong Container, no image foundSPL: failed to boot from all boot devices"
These are the files that I use the generate the .bin file: mx8ulpa2-ahab-container.img, upower_a1.bin (fimware-upower-1.3.0), bl31.bin , u-boot.bin, u-boot-spl.bin and the m33_image.bin. Used command "make imx8ulp_evk_defconfig " to generate the u boot files.