You have a stream named in3 which is only used for output.

If there are no lines now in the widget you fail to truncate/clear out any lines which were in the file (e.g. after deleting).

Etc.

It makes no sense. I leave it to you.

(https://ddgobkiprc33d.cloudfront.net/6e977f62-7100-4a2e-93c0-bbea3b949e80.png)

void MainWindow::on_add_clicked()
{
    QString fname1st = ("prac3.txt");
    QFile file1st(fname1st);
    if(file1st.open(QIODevice::WriteOnly | QIODevice::Append | QIODevice::Text))
    {
        QTextStream stream(&file1st);
        QString file1st = ui->lineEdit_2->text();
        stream << file1st <<"\n";
    }
}

on my code i used:

void MainWindow::on_pushButton_2_clicked()
{
//update listwidget
        QFile file3("prac3.txt");
        file3.open(QIODevice::ReadWrite | QIODevice::Append | QIODevice::Text);
        QTextStream in3(&file3);

        QString line1 = ui->comboBox->currentText();
        QString amount = ui->lineEdit->text();   //number of item text
//detele old item data
        QString search(line1);
        QList<QListWidgetItem *> list = ui->listWidget->findItems(line1, Qt::MatchContains);
        for ( QListWidgetItem *item : list )
        delete item;
//add updated item with amount
        QStringList stringList;
        stringList << line1+"\t"+amount;
        ui->listWidget->addItems(stringList);
//Update the text file
        auto numRows = ui->listWidget->count();
        for(int row = 0; row < numRows; ++row)
        {
            auto item = ui->listWidget->item(row);
            QString s;
            s = item->text();
            file3.resize(0);   // Delets all data from the textfile
            in3<<s;            //adds row to text file
            in3<<"\n";       //adds a line break
        }
        file3.close();
 }

//Update the text file
auto numRows = ui->listWidget->count();
for(int row = 0; row < numRows; ++row)
{
auto item = ui->listWidget->item(row);
s = item->text();
file3.resize(0); // Delets all data from the textfile
in3<<s; //adds row to text file
in3<<"\n"; //adds a line break
}
file3.close();

Whether you add, delete or modify, to update the text file you need to rewrite the whole thing, whenever you wish to do so. You need to get rid of whatever you are trying to do in your code with resize(). You need to close your file after reading it in, and open (with truncate/overwrite) any time you want to write all the current data out (and of course close it when done writing it). That is how text files are updated.