My ceiling metal junction box has 2 white wires(twisted together) and 1 black wire. No copper or ground wire inside that I can see? The new fixture has a black, white and green/yellow ground wire. The provided new metal mounting strap bracket does not have a green ground screw. What can I attach the green/yellow ground wire from the fixture to??
Although 2 whites and a black would be unusual. In the US code allows for a grounding conductor to be pulled from the service, or any point fed from that service panel.
2 wire fixtures did not have grounds for the first 75 years of electrical code so many fixtures were not grounded.
If fed by conduit and individual wires the box may be grounded through the conduit.
All metal parts has to be grounded. Check carefully your junction box, bare copper wire should be connected to screw inside the box. You can connect the ground wire from your fixture to same screw or another screw in the box. If you have wiring in your house with ground wire, the metal box body is ground. If the wire from fixture too short, make a jumper. If it is no ground wire, leave the fixture ground wire not connected or may connect it to box screw. Two white wires connected together because the neutral is jumped from that box to another. Connect white wire from fixture to them. An black to black.
I assume that the two white wires are two lives are for two separate circuits (i. e. two lights, switchable seperately or light+ceiling fan) and the black wire is neutral. In this case, you would connect the ground to the neutral. That's the normal way to do this if you have no ground. However, I would first check what the incoming wires actually connect to. Also you can't have an RCD with this kind of wiring because it would blow the RCD immediately if you touch the grounded parts of your new fixture.