QIODevice::write (QFile, ":\data.txt"): device not open. ReadOnle на WriteOnly поменял,
QFile text(":/data.txt");
text.open(QFile::WriteOnly |QFile::Text | QFile::Append);
if (text.exists()){
ui->label->setText("OK!");
QTextStream stream(&text);
stream<<"slovo";
}
text.close();