To unlock the Just a Scratch trophy in Red Dead Redemption 2 you simply need to complete a very early story mission in Chapter 1. It will unavoidably be the first trophy you unlock.
*** Spoiler - click to reveal ***After rescuing Sadie Adler from the O'Driscolls in the opening sequence, you will be tasked with riding out to find John Marston with Javier. Bring him back to camp and you will get this trophy.