So, my mistake was starting playing Forza Horizon 2 late 2016, which meant Storm Island was available since the beggining, so I just entered Storm Island as soon as I could (before being introduced to Barn Findings on the base game) and went there and fully completed all the championships which meant that, since I wasn’t introduced to the mechanics of Barn Finds, I didn’t get any message about a Barn Finding on Storm Island… Now I just cannot get it… I have already completed the Horizon Finale on the base game (and decided where the Road Trip should go) and so I decided to go back to Storm Island… I have already done all the championships again and I just don’t get any message of the Barn Finding on this island, I even went manually to the location of that Barn and the video just doesn’t trigger… Am I doomed to never complete the Storm Island achievements?? 
You’ll probably just have to restart the game and get it from there, making sure to unlock barn finds in the base game before traveling to Storm Island. If that isn’t the case, though, just drive around Storm Island for a while and see if the notification pops up. A certain number of hours and miles driven on Storm Island is needed to unlock the 6R4. The stats cannot be increased on the main map, you have to drive around on Storm Island.
Hi sir and thanks for your time!!
When you say so, you mean I need to delete any save file from my Xbox One right? and needing to do everything again? I don’t mind that much (I just finished the base game 'til the credits and stopped playing so I will not care if I have to re do the mains story)
Yes, that’s it. This has happened a few times before, namely in the last few months. Delete the save from both your console and the cloud. You’ll lose all progress but you’ll be able to get the 6R4. Just make sure to unlock barn finds within the main game even before traveling to Storm Island. (Better yet, to be safer, find the first barn find in the main game and then travel to Storm Island.)