Hi @CallMeAlex,
If you are using Shopware 6.1 or above, use the action routes to update the order status (you can find the link above) and not by patching the order’s state. The error that you’re getting might be related to the fact that you can’t change the order state from ‘cancel’ for example to ‘done’, first you need to ‘reopen’ the order in order to mark it as ‘done’. Try using another state.