The technology "Advanced Manganese Smelting 3", the one unlocking the Pellet from the Processed manganese, is missing a recipe to convert it to manganese cathode 2.
The manganese Oxide item exist but can't be produced.
The technology is only modded with angel's smelting and seablock 2 so i think you're missing a recipe.
After reserching online, the recipe missing is :
Chemical Furnace = Manganese Pellets + Natural Gas → Manganese Oxide
