"No options defined for block variation"

[RobMcDesign]

I'm trying to get a variable axis, I've followed the example from
https://underscoretype.github.io/fontsampler-js/variable.html

I've added the axes and instances to the font var
name: "VariableFontName",
files: ["VariableFontFile.woff2"],
  axes: ["wght"],
  instances: [
  "wght 500, | VariableFontName Regular",]

I've added a variation element
order: ["tester", ["fontsize", "variation", "fontfamily"]],

and I've tried to define that element
variation: {
axes: [{
tag: "wght",
init: 400,
min: 250,
max: 900,
step: 10,
label: "Weight"
}]
},

but I'm still getting the error. Am I missing something? I couldn't find any more documentation on variable font support.