não consigo identificar o erro

There are some syntactical problems. You try to work with instance variables (at least I assume so as you use the keyword this) but you didn't declare one. You probably meant to be the vareiables in line 9 and 10 to be such variables but they are just local ones. That's one reason why initializing these vars in lines 12, 13 fails. The second reason is that you can not access instance members from a static context (static method). You'd need to create an object first. Lines 15 and 16 have two porblems. 1. they are outside a method and outside methods you just can declare static and instance variables oo declare and initialize them or you can have static and instance blocks. 2. inside an if condition you can not declare variables. Even if it would be possible you haven't assigned a value (initialized it). That way you would just have reserved memory for the variable but it is unknown to the compiler and it can not check if that not known value is greater than zero. In addition to that the condition is useless as it has an empty body (a semicolon after it) and the following return statement does not belong to that condition anymore and is executed in any case. Line 17 is even outside of the class (after the closing parenthesis of the class level) and the return value not defined. Better follow the learning path and understand what you are doing.

public class Solution {
    private int ano;
    private int idade;

    public Solution(int ano, int idade) {
        this.ano = ano;
        this.idade = idade;
    }

    public int nasc() {
        return (ano - idade);
    }

    public static void main(String[] args) {
        System.out.println(new Solution(3126, 8).nasc());
    }

}

Na proposta desse exercício, acredito que nem precisava trabalhar com método, mas acredito que o erro possa ser pq o método está definido fora da main ou por não estar sendo chamado na main, não tenho certeza, porem é o que eu tentaria testar primeiro. Mas poderia ter feito um simples "System.out.println(3126 - 8)" que resolveria o problema também