Mostly we use che for that or which. But in more formal contexts you can say the which using il quale , la quale , i quali , le quali. (Changes in number but not gender)

the which likes prepositions: of the which, to the which

This sentence "I do not have the books of the which we speak" starts Non ho i libri di i quali parliamo

Contracting the preposition: Non ho i libro dei quali parliamo.

An alternative is to use cui with the preposition: Non ho i libri di cui parliamo. cui is invariant.

I am for cui

These lessons really stress test the Duo method. I expect lots of people give up here.

I went with the more informal English sentence of "I do not have the books that we talk about."

Stay positive guys!! What about "I do not have the books WE ARE DISCUSSING"??

Well, I mean,,, if "I don't have the books that we are talking about" is perceived as a wrong answer, then the given answer is also wrong, because no sentences should end with any prepositions. If you REALLY REALLY want to be pathentic, then it should be "I do not have the books of which we are speaking". :/ No?

Now its teaching me English grammar - my only error was to use that instead of which. I don't even know if they are right!