You could make a variable outside of the for-loop that is a reference to the scene:
let myScene;
create() {
myScene = this;
for (var a=1; a<=10; a++) {
var mybutton = this.add.image(60*a, 30, "btnLevel").setInteractive();
mybutton.myid = "btnLevel" + a;
mybutton.on("pointerup", function (e) {
myScene.scene.switch("sceneNext", {
"level": this.myid});
});
}
}