Hi @HotHanni ,
This is currently expected behaviour, as at this time product unit prices are not available to be edited or created via our API. They can however be retrieved from the API on existing orders through the Order.lineItem.unitPrice field.
They can also be edited in the Shopify Web Admin for stores located in Germany or France by default, or if your store is located elsewhere you can reach out to Shopify Support to enable the feature, though this request must come from the store’s account owner.
I do understand how this is not ideal for your business operations, and I will be sending up some feedback on your behalf that you would like to see Unit Prices manageable via the API directly. We do take all our merchants and partners feedback with great value, and the more feedback we get for features like this, the more likely and quicker it will be added!
I hope this helps, and i hope you have a great day 