Is it possible to replay the mission Freedom of the Press in Assassins Creed Syndicate if it says 0%?
If the memory is listed as 0% sync, that means you haven't finished it yet and thus you can't possibly replay it (you automatically get 50%-100% sync by completing the mission, depending on its bonus objectives). This particular mission can only be completed once you've cleared the rival gang out of the Whitechapel area, so make sure you've done that before trying to find it. Then you'll need to find the mission start on the map and go there to play the mission.
In addition to animuson's answer:
In order to replay missions you have already completed - whether or not the bonus objectives were met - you have to navigate to the 'Progression Log' (at the top in the main menu), then the 'Progress Tracker'. There, you can choose the category of the memory (Story/Associate Activities/London Stories), and then its subcategory (Sequence/Activity/Story). Neatly convoluted (as Ubisoft likes it).
There you can select the mission in question, and choose to replay it.