Robot pain is theoretically possible
Our current understanding of pain is incoherent and self-contradictory, but once we have a coherent theory of pain, a robot could in principle be constructed to instantiate that theory and thereby feel pain.
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Daniel Dennett, 1978.
Dennett cites as an example of the incoherence of our current understanding of pain the observation that morphine is sometimes described as preventing the generation of pain and sometimes as just blocking pain that already exists.