Hi @Stuswi ,
Could you please take a picture of this using a different device and share it here using the Insert photo option? Is this visible in all the apps and screens?
When this happened to my Samsung Galaxy s⁷ Edge it turned out to be the connection between the display and motherboard, and subsequently needed the display and connector replaced which was carried out under the 2 year warranty.
My line was pink.
You could try the colour panels by launching your phone app and dial *#0*# and rotate through the panels to see if that helps.
Ultimately though you'll most probably need to ask the provider of the phone for support or ask a Samsung Service Centre for support under the manufacturers warranty as long as the phone displays no sign of damage.
In the UK a person has 28 days to return a phone if purchased directly from Samsung UK but a network may have their own returns period.
For your own region please check any small print.
I wish you all the best. 
@Briff29: If you dial *#0*# into your telephone keypad and select Red, Green and Blue from the series of grey tiles, is the line present in any of these tests? If so, and if you are based in the UK or Ireland, please reach out to our Help Desk via the link below, and an advisor will be able to arrange an engineers inspection for you.
https://www.samsung.com/uk/support/contact/
If you purchased your Galaxy s21 through a mobile network contract then please be assured that it is covered by our standard warranty, which automatically activates on the purchase/contract start date, runs for 24 months, and covers software and hardware defects. If physical or liquid damage is identified during the engineers inspection, which is not covered, then a charge will be applicable. Don't worry though, as any costs will be communicated to you before any repairs are carried out, allowing you the chance to opt in or out of the service.
