If java similar to c# then why can't use it with unity engine

Answer 1

Java's and C#'s similarities ends with the source code. They compile in different bytecodes, stored in different file types, that will be loaded by different virtual machines that will compile them in totally different machine code instructions.

Answer 2

I learned Java. Then decided to learn Unity and C#... turns out thanks to Java I knew enough C# to ignore it and just learn the Unity stuff to go with it