te recomiendo arreglar en formato json (javascript object object notation), tambien olvidaste cerrar el ajax
function login(){
var username = $('#username').val();
var password = $('#password').val();
$.ajax({
url:'../Controllers/user.php',
type:'POST',
data:'username='+username+'&password='+password+"&boton=login"
}).done(function(resp){
if(resp=='0'){
$('#error').show();
}else{
location.href='../Views/main.php';
}
});
}
cambia a
function login(){
var username = $('#username').val();
var password = $('#password').val();
$.ajax({
url:'../Controllers/user.php',
type:'POST',
data:{username:username, password:password, boton:"login"},
done:(function(resp){
if(resp=='0'){
$('#error').show();
}else{
location.href='../Views/main.php';
}
})
});