Lecture notes or video lectures on generalised symmetries of differential equations. Currently, I am reading the chapter 5 from the Application of Lie groups to differential equations by Peter J.  Olver. Unfortunately, I find it difficult to follow. Could you please suggest some video lectures or lecture notes on the same. Thank you for your help and time.

Olver's book is a bit advanced for first learning Lie theory applied to differential equations; cf. this answer. Try Bluman & Anco's Symmetry and Integration Methods for Differential Equations (2002).
Also, see Schwalm's lecture notes: Lectures on Selected Topics in Mathematical Physics: Introduction to Lie Theory with Applications (2017), to be accompanied with online video lectures (as he did for his book on elliptic function solutions to ODEs).
