I suspected that the tape made it so that the nut didn’t tighten down enough to compress the Ferrule all the way. I would first try removing all the tape and then re-tightening the nut and then checking to see if you still have a leak. Then if you still have a leak you would need to replace the ferrules.
For replacing the Ferrule , Usually once the Ferrule is compressed onto the pipe it requires cutting off the end of the pipe with a ferrule. If you could pull the ferrule off at the pipe then it wasn’t compressod properly.
The nut should be fine only the ferrule should need to be replaced. In my experience Ferrules need much tighter compression than what hand tightening would supply. Used two wrenches, one on the Valve to keep it from spinning and one to tighten the nut that compresses the ferrule to the copper pipe. Tightening The nut i’m to the valve pulls the valve towards the nut with the feral in the middle thereby compressing the ferrule to the copper and squeezing the valve and the nut together on the Ferrule, which makes it a watertight seal if done properly.