On your game screen - look to the left see how many Tech Points you have - Construction Level 2 costs 2500 Tech Points -- Farming Level 2 costs 5000 (or is it 6000?) ... if you have enough tech points click on Tech at the bottom left and Buy Farming or Construction --- by the end of the game to complete all the puzzles you almost need Level 3 in everything

If your tech points are still at 99..... you need to drag some of your villagers to the table in the middle of the village and teach them to research -- they need to "learn" how to research and this is the only way to accrue tech points....