Not all assets are fractionable yet so please make sure you query assets details to check for the parameter fractionable = true.

Supported fractionable assets would return a response that looks like this

  "id": "b0b6dd9d-8b9b-48a9-ba46-b9d54906e415",
  "class": "us_equity",
  "exchange": "NASDAQ",
  "symbol": "AAPL",
  "name": "Apple Inc. Common Stock",
  "status": "active",
  "tradable": true,
  "marginable": true,
  "shortable": true,
  "easy_to_borrow": true,
  "fractionable": true

If you request a fractional share order for a stock that is not yet fractionable, the order will get rejected with an error message that reads requested asset is not fractionable.