देवैरत्रापि विचिकित्सितं पुरा न हि सुविज्ञेयमणुरेष धर्मः ।
अन्यं वरं नचिकेतो वृणीष्व मा मोपरोत्सीरति मा सृजैनम् ॥ २१ ॥

devairatrāpi vicikitsitaṃ purā na hi suvijñeyamaṇureṣa dharmaḥ |
anyaṃ varaṃ naciketo vṛṇīṣva mā moparotsīrati mā sṛjainam || 21 ||

21. Here, even the gods of yore had doubt. Indeed it is not easy to know—subtle is this matter—Oh, Nachiketas, ask for some other boon. Press not this on me; give this up for me.

Shankara’s Commentary:

Death in order to test whether he was or was not absolutely fit to acquire the knowledge of the atman leading to emancipation, said this: ‘Even by the gods in older times, doubt was entertained on this point. It is not easy to be known, though heard explained, by ordinary men. Because this subject of the atman is subtle. Therefore, Oh, Nachiketas, ask for another boon, whose fruit is certain; press me not as a creditor presses a debtor. Give up this boon for me.