Is Friday Prayer valid if performed standing behind an imam who is sitting on a chair?

The Imamate of a seated person for a standing person is valid unconditionally according to Abu Hanifa and Al-Shafi'i. The Hanbalis stipulate that he must be the Imam of the neighborhood and that there is hope for his ailment to cease. According to the Hanbalis, the follower prays while seated, and if he prays standing, his prayer is valid according to one of the two views in the madhhab. Your prayer behind the aforementioned Imam is valid.