Why can't I keep fish alive...

[Black Phantom]

You need to give us a bit more information. And don't always trust a test kit. Borrow or buy a different brand for ammonia and try that. What kind of water are you using. What sort of symptoms do the fish show before they die.
Check your RO filters. Are they clogged and letting chlorine and other chemicals in.
Do a chlorine test.
Hope some of this helps