Property ‘getContext’ does not exist on type ‘HTMLElement’

I would try:

const canvas = <HTMLCanvasElement> document.getElementById('myChart');
const ctx = canvas.getContext('2d');

the purpose of Typescript is to avoid wrong types. By default document.getElementById returns a HTMLElementtype which is a generic type.
In order to make your app understand it is a canvas element you need to cast it using <CastedToType> syntax.