The chart seem to be async so you will probably need to provide a callback when the animation has finished or else the canvas will be empty.

var options = {
    bezierCurve : false,
    onAnimationComplete: done  /// calls function done() {} at end
};

its not possible to render without attaching canvas to dom, but you can hide it and it will work as expected.

const chartEl = document.createElement("canvas");
chartEl.setAttribute("width", "400");
chartEl.setAttribute("height", "400");
chartEl.style.display = "none";

document.body.append(chartEl);
let chart = new Chart(chartEl, {
  type: "bar",
  data: {
    datasets: [
      {
        barPercentage: 0.5,
        barThickness: 6,
        maxBarThickness: 8,
        minBarLength: 2,
        data: [10, 20, 30, 40, 50, 60, 70]
      }
    ]
  },
  options: {
    scales: {},
    animation: false
  }
});

chart.render();
console.log(chart.toBase64Image());
chartEl.remove();