My playstation keeps saying to put in a memory card.. even though I have done

Did you try reading the memory card without any game discs put in via the ps2 menu?

is your memory card working properly? could be a corrupted memory card or a faulty memory card slot.
try putting a friend's memory card on that same slot.
if it works... then your memory card is gone bad...
if it doesnt work...then your memory card slot has gone bad.

else try putting your memory card in your friend's ps2, see if that ps2 reads your memory card. if it does...then your memory card slot has gone bad.