If you are trying to connect the isolation valve (chrome coloured) to the short piece of copper tube with the earthing strap just below the brass nut (right hand side of your second picture), then:
1. As just pumps has said, you need an "olive", not a washer.
2. An olive is just a ring of brass or copper which fits over the pipe. The outside of the ring is slightly domed. When the nut is done up, it squeezes the olive between a chamfer on the inside of the nut and another on the inside edge of the fitting. This squeezing seals the inside of the olive onto the pipe and the outside onto the fitting, creating a water tight seal.
3. There may well be an olive remaining inside the brass nut on the copper pipe. If you can't pull that brass nut off the pipe with gentle pressure, the chances are the olive is still in there.
4. If the olive is still there, try doing the joint up again, making sure all is square as you screw the nut onto the thread of the isolation valve. If it still leaks, replace the olive.
5. If you have to replace the olive, you'll have to remove the old one. There are various ways of doing this, but:
5a. Make sure you don't damage the pipe or the new olive may not seal.
5b. You may have to move the earthing strap down to get at the olive. Don't forget to fasten it up again tight when you have finished.
6. If the olive is missing, remove the nut, clean up the pipe, replace the nut and tighten up with a new olive.
7. If you can get copper olives, they are softer than brass ones, and easier to get tight. However, its not that important, so use brass if that's all you can get.
