You're right, I cannot force you to agree the sky is blue. That doesn't necessarily mean there is a defect; maybe there is a defect in the method of argumentation.

It seems that the tortoise is revealing a problem with deduction, specifically modus ponens. In order to accept Z you must accept modus ponens (C). Then in order to accept Z you have to accept modus ponens (D). It seems that we must use either circular reasoning or an infinite regress to support modus ponens.