Tafsir of Al-Asr 103:2

(Indeed, mankind is in loss), meaning a loss in their dealings, their endeavors, and the expenditure of their lifespans in their pursuits from which they derive no benefit in the Hereafter; rather, they may perhaps cause them harm when they reach the Sahirah (the resurrection ground).

The definite article (al- in al-insan) is for the purpose of encompassing all (generality), supported by the context of the exception that follows. Regarding the indefiniteness of khusr (loss), it is said to be for glorification—that is, in a great loss. It is also permissible that it is for categorization, meaning a type of loss other than what man recognizes.