I following the instruction of jawn-smith " [Installing Ubuntu 21.04 on the HiFive Unmatched]" on nvme and everything work fine but the SD card is needded to boot.
Can I use the Boot Mode Select DIP Switch to boot without sd card ?
The is all explained in the Booting from Flash thread. Not clear why you are asking here. The flash is just another disk with partitions like the sdcard, and the boot select switches on the motherboard determines which disk we read from at boot.