I have sewed sackcloth upon my skin. — Referring, probably, to the state of his skin, which had become hard and rugged as sackcloth. As the second half of the verse must be figurative, there seems to be no reason to understand the first half otherwise.
Related Commentaries of Job 16:15
Job 16:15
15 I have sewed sackcloth upon my skin, and defiled my horn in the dust.