Assalam aalekum I have in musanaf ibne abi shaybah the lowest person in jannah will have 1000 palaces is this Hadith authentic

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The report you mentioned is not a Hadeeth from the Prophet but is a statement from Ibn 'Umar (i.e., Mawqoof).

Ibn Abu Shaybah reported in Al-Musannaf that Ibn 'Umar said: "Indeed the least of the people of Paradise in rank is one who has one thousand palaces, the distance between each palace and the one after it is one year, he can see the farthest part just like can see the closest one, there are Houris (ladies of Paradise), scented plants, and young boys, and whatever he asks for is brought to him."

Al-Albaani classified it as inauthentic in his book Dha'eef At-Targheeb wa At-Tarheeb (2186). He based this classification on the fact that one of the narrators, named Thuwair, was problematic. He said that his name is Ibn Abu Fakhitah, who was weak and was considered a lair by some scholars.

However, Al-Albani classified the report of Ibn Abbaas as authentic, which states that Allah Almighty granted His Prophet one thousand palaces in Paradise. This was also reported by Ibn Abu Shaybah and At-Tabaraani.

Allah Knows best.