I know something is missing, I just don't know how to fix it/what to add.

Loads of errors for a little code ...
You are declaring Double inside the Main method - you can do that, but you need to lose the semicolon at the end of the definition line, and replace it with curly brackets around the code that is inside the Double method

Because Double is a local function to the static main method, it needs to be declared as static as well.

To execute Double, you need to explicitly call the function and provide data in the form of parameters.

Pick a bracketing / indentation style, and be consistent with it: you have mixed two styles so it gets a lot harder to work out what is going on. It's not a major problem with a trivial piece of code like this, but as your code grows, it gets exponentially harder to see what is causing problems.

Try this:

public class Main
{
  public static void main (String args[])
  {
    System.out.println (multiply (14.2, 67.9));
  }

  static Double multiply (Double a, Double b)
  {
    return a * b;
  }
}

You should expect to get syntax errors every day, probably many times a day while you are coding - we all do regardless of how much experience we have! Sometimes, we misspell a variable, or a keyword; sometimes we forget to close a string or a code block. Sometimes the cat walks over your keyboard and types something really weird. Sometimes we just forget how many parameters a method call needs.

We all make mistakes.

And because we all do it, we all have to fix syntax errors - and it's a lot quicker to learn how and fix them yourself than to wait for someone else to fix them for you! So invest a little time in learning how to read error messages, and how to interpret your code as written in the light of what the compiler is telling you is wrong - it really is trying to be helpful!

So read this: How to Write Code to Solve a Problem, A Beginner's Guide Part 2: Syntax Errors[^] - it should help you next time you get a compilation error!