Hard to tell with so little information. This tutorial is very interesting in my opinion. The effect on the other hand is not so simple, there is a lot of room for making mistakes along the way.
I suggest to check whether the RT-texture behaves the right way first. If you are 100% sure it does, you could try changing the camera mode from orthogonal back to its original settings, but I doubt that would make a difference.
There is just too little information to tell whats going on. Also it would help if you were showing the artifacts you are getting.
Other than that I can only recommend to check the tutorial and do it step-by-step to make sure everything is working correctly. Starting all over again does help sometimes even tho it takes a lot.
EDIT: I noticed that they are doing it completely different than you, is there a reason you are not following the tutorial on this part when it seems to work according to the tutorial?
It is Unreal Engine 4, but should be the same for UE5 in my opinion.