Method Illuminate \ Database \ Eloquent \ Collection :: whereHas does not exist

You see, I have to start a table Sector :

Schema::create('sectors', function (Blueprint $table){
        $table->increments('id');
        $table->string('nombre');
        $table->timestamps();
});

This table is used in a table called Experiencia . They have no formal relationship, but the value for sector_empresa is taken from the previous table:

Schema::create('experiencias', function (Blueprint $table) {
        $table->increments('id');
        $table->string('puesto');
        $table->string('funcion_realizada');
        $table->string('empresa');
        $table->string('sector_empresa');
        $table->string('mes_anyo_inicio');
        $table->string('mes_anyo_fin');
        $table->unsignedInteger('user_id');
        $table->foreign('user_id')->references('id')->on('users');
        $table->timestamps();
    });

And this is related to the users of the table:

Schema::create('users', function(Blueprint $table){
        $table->increments('id');
        $table->string('name');
        $table->string('second_name')->nullable();
        $table->string('provincia')->nullable();
        $table->string('localidad')->nullable();
        $table->string('direccion')->nullable();
        $table->string('telefono');
        $table->string('dni');
        $table->boolean('vehiculo')->default(false);
        $table->string('foto')->default('anonimo.jpg');
        $table->boolean('activado')->default(false); // Se marca aqui como falso y ya se pone automaticamente así al hacer el formulario.
        $table->string('email')->unique();
        $table->string('password');
        $table->rememberToken();
        $table->timestamps();
    });

The story is that I created a function to show, for a certain sector, if a user has experience in that:

public function fichaje(Sector $sector){
        $usuarios=User::all()->whereHas('experiencias',function($query){
            $query->where('sector_empresa',$sector->nombre);
        })->get();

        if(count($usuarios)==0)
            return back()->with('message',['danger','No hay ningun usuario interesado en este sector']);
        else
            return back()->with('message',['success','Hay usuarios interesados en este sector']);
    }

But when I run the code it gives me this error:

What will I be doing wrong?

Edit: I have removed the "all ()" from the conditional, thus leaving the code:

public function fichaje(Sector $sector){
        $usuarios=User::whereHas('experiencias',function($query){
            $query->where('sector_empresa',$sector->nombre);
        })->get();

        if(count($usuarios)==0)
            return back()->with('message',['danger','No hay ningun usuario interesado en este sector']);
        else
            return back()->with('message',['success','Hay usuarios interesados en este sector']);
    }

But now I get a new error message:

It seems that what is in the function does not capture the variables from outside.