Your LTW is level 15. The Buffalo Leather synth is around level 28. You cannot attempt synthesis more than 10 levels over your level.

"Items not used in new recipes" - At one point during the game's life, they made a large number of changes to the crafting system, one of which was to totally revamp most of the recipes. Buffalo leather was one of the items/synths that got the axe, and when 2.0 hits, the ability to craft such "dated" recipes will be removed.