Cant make an Update, we could not complete your purchase. install_error_distribution_signed_by_apple

hello there

i just bought a used macbook pro 6,2 i5 using 10.6.8

im trying to update my OS x but the software update says im up to date and the app store is working weirly ( showing pics and app on each other, not the proper way) and if i try to download Yosemite from the app store it give me a message '' we could not complete your purchase. install_error_distribution_signed_by_apple''

so what should i do??