Hi Thasina, you might want to try rebooting your phone to see if the line goes away. If that fails, you can bring it to the service center to see what they can do for you. Hope this helps!
Aiyo.. why their updates keep breaking 2020 flagships.. first, was S20+, then S20 Ultra, followed by Note 20 Ultra and now ZF2. Somemore, all came from 2020. Seems like there is a manufacturer defect for the 2020 OLEDs. Wonder if they actually outsourced the display parts from external suppliers to save cost..