Thin red line on the side of M1 MacBook Pro display.

Contact Apple Support and/or make a Genius Bar appointment at an Apple Store. That is indicative of a hardware issue. It should be covered under warranty.

Bad_Life_Choices wrote:

On the right side of the display, there is a thin red line going from top to bottom. It is right beside the scroll bar in a browser. When taking a screenshot, the red line doesn't appear within the screenshot. I restarted the device too. The red line just appeared this morning. Last night, it wasn't there. When I woke up it wasn't there. It just suddenly appeared. Also, this laptop hasn't sustained any damage and the laptop has never been opened.

Call Customer Support (800) MY–APPLE (800–692–7753)

or on line https://getsupport.apple.com/

or call AppleCare Support at 1-800-APLCARE (800-275-2273)

Your Mac ships with a one-year warranty...

https://support.apple.com/my-support

Learn how to get your Mac fixed and how much it will cost.

https://support.apple.com/mac/repair/service

Get your Mac ready for service - Apple Support https://support.apple.com/en-us/HT209095