If you're accepting orders via POS, there's no actual user account, so you need to 'mock' one, as Fakturoid requires each invoice to have a customer. To support POS orders with no customer, create a new one in your Fakturoid account:
Name it ‘Retail Sale’ or anything you prefer – these customer details will be visible on the invoice.
When you open the customer details, you’ll see a browser URL like this:
https://app.fakturoid.cz/jmeno_uctu/subjects/12933447
Copy the number (customer ID) from the URL to the app (in this case, 12933447).