Why did Jesus say to God: "nor do I know what is in Your Self," and is it valid to describe God as "Self," and what is the implication of this word?

It is permissible to use the phrase "Nafs of Allah" (Allah's Self), based on the saying of Jesus (peace be upon him), and Allah's statements: "And Allah warns you of His Self" (Quran 3:28), and "Your Lord has decreed upon Himself mercy" (Quran 6:54), and the Hadith Qudsi: "Indeed, I have forbidden injustice for My Self," and "Whoever remembers Me to himself, I remember him to My Self," and the Prophet's (peace be upon him) saying: "Glory be to Allah, the contentment of His Self."

The Nafs of Allah Almighty is His Essence, as stated by Al-Hafiz Ibn Hajar and Shaykh al-Islam Ibn Taymiyyah. It is not an essence separate from attributes, nor an attribute of the Essence.