Hey @AlexeyKhimich ,
Welcome to the Shopify Community!
It sounds like you are attempting to change the financial status of an order to “Paid” through the API, but you are experiencing an error.
It is not uncommon for certain actions to be restricted when using the API, and it is possible that the ability to manually mark an order as “Paid” is only available through the Shopify admin panel and not through the API.
One option you could try is to use the Transactions API to create a transaction on the order and set the kind field to 'sale'. This would simulate the order being paid and could be a way to achieve the desired behavior.
You could also consider using the Draft Order API to create a draft order and then use the Order Editing API to make changes to the draft order, including setting the financial status to “Paid”, before it is finalised. This would allow you to update the order through the API.
You can learn more via our Shopify Developer guides about getting started with authentication and building apps.
Let me know how you get on!