Dedicated to the Hydrogen framework, headless commerce, and building custom storefronts using the Storefront API.
@laurilawliet can please post the full detail
can you please post what data you passing?
I have no issue with this customer API and data it work form me
API Endpoint
https://key-XXXXXXXXXXXXX3dba1:shppa_xxxxxxxxxxxxxx@your-store.myshopify.com/admin/api/2021-10/customers.json
{"customer":{"id":"ab45454","email":"testcustomer123@gmail.com","phone":"4564564566","first_name":"first","last_name":"last","created_at":"2020-01-08T12:01:08.000Z","updated_at":"2021-11-01T11:20:52.000Z","accepts_marketing":true,"note":6,"orders_count":0,"state":"disabled","total_spent":"0.00","last_order_id":null,"verified_email":true,"multipass_identifier":null,"tax_exempt":false,"tags":"api_user","last_order_name":null,"currency":"USD"}}
{