Why after the software update.. the screen shows a green line
This is a known issue especially for S20 series. You can search more about it from this community page. I do heard that Samsung is giving a free screen replacement to S20 series user who is facing such issue but with some terms.
But for Note 20, i think they will charge for it.
Feel free to chat or visit to their service center for more information.
FYI, here is the price for screen replacement from Samsung and their contact page:
** If i am not mistaken Note 20 is using Exynos chipset in Malaysia (i might be wrong). If you are using a Snapdragon chipset, the replacement might be a bit difficult because the device brought is not from Malaysia. Anyways, you can go check with Samsung support.