Hi Sebastián,

what does your request look like? When looking at the cURL example in the docs (https://woocommerce.github.io/woocommerce-rest-api-docs/?shell#create-a-product) you need to pass the category IDs like this:

"categories": [
  {
    "id": 9
  },
  {
    "id": 14
  }
],

So it is an array of objects.

Best,
Florian