Hi sung ,
Could you try with another HDMI cable? (I had many trouble with the one provided by Beelink)
FYI I just tested the "LibreELEC-H6.arm-9.80-nightly-20200222-e49037d-beelink-gs1.img.gz" with LibreELEC USB-SD and everything works fine.
i had tried 3 different hdmi cable. each tv and monitor have different hdmi cable. but still just black screen