<canvas width='600' height='400'></canvas>
<script>
var tela= document.querySelector('canvas');
var pincel= tela.getContext('2D');
pincel.fillStyle='lighgrey'
pincel.fillRect(0, 0, 600, 400);
</script>
Solucionado
<canvas width='600' height='400'></canvas>
<script>
var tela= document.querySelector('canvas');
var pincel= tela.getContext('2D');
pincel.fillStyle='lighgrey'
pincel.fillRect(0, 0, 600, 400);
</script>
Rodrigo, de boa ?
Precisa ser minusculo quando faz o 2D, então ficaria assim: var pincel= tela.getContext('2d');