Or HaChaim on Exodus 33:20/אור החיים על שמות ל״ג:כ׳

1 ויאמר לא תוכל לראות, He said: "you are not able to see, etc." G'd meant that it was not He who withheld visual insights from Moses, it was simply that being a mortal human being, a composite of flesh and spirit, made such a thing impossible. This is why G'd added the word וחי, "and remain alive," to explain to Moses that because G'd wanted Moses to remain alive He could not grant his wish at this time. This would correspond to the thoughts expressed by the Psalmist when he said: (Psalms 84,11) "better a day in Your courtyard than a thousand (days in Your palace)."

2Alternatively, we may understand this verse to mean that even if man were חי, i.e. a perfectly righteous individual, immortal, G'd cannot be "seen" by man. We would have to understand the word חי in the sense it is used in Samuel II 23,20 where Benayahu son of Yehoyadah is described as בן איש חי, "a perfectly righteous individual." The letter ו at the beginning of the word וחי in our verse indicates that it adds something to what had already been stated, i.e. the inability to "see."

3Still another way of understanding the words כי לא יראני האדם וחי, is that even after death it is impossible for man to "see" G'd. G'd asks: How can you expect to "see" G'd while still alive? Whatever "light" the righteous will be able to "see" in the hereafter does not come close to what you Moses are asking of Me at this time.