Array to string conversion error on sql insert in laravel 5.6

I'm trying to insert into order_items table on laravel 5.6 and getting following error

"Array to string conversion (SQL: insert into order_items (title, order_id, quantity, unit_price, unit_booking_fee) values (sector1, 129, 1, 1, 1))"

order_items table schema

    Schema::create('order_items', function ($table) {
        $table->increments('id');
        $table->string('title', 255);
        $table->integer('quantity');
        $table->decimal('unit_price', 13, 2);
        $table->decimal('unit_booking_fee', 13, 2)->nullable();
        $table->unsignedInteger('order_id');
        $table->foreign('order_id')->references('id')->on('orders')->onDelete('cascade');
        $table->softDeletes();
    });

I've tried following both code and both give same error

        $orderItem = new OrderItem();
        $orderItem->title = $attendee_details['ticket']['title'];
        $orderItem->quantity = $attendee_details['qty'];
        $orderItem->order_id = $order_id;
        $orderItem->unit_price = $attendee_details['ticket']['price']; //0.15
        $orderItem->unit_booking_fee = $attendee_details['ticket']['booking_fee'] + $attendee_details['ticket']['organiser_booking_fee']; //0.01
        $orderItem->save();


        OrderItem::create([
            'title' => 'sector1',//$attendee_details['ticket']['title'],
            'order_id' => $order_id,
            'quantity' => $attendee_details['qty'],
            'unit_price' => 1,
            'unit_booking_fee' => 1
        ]);

What am i doing wrong?