Error: Creating a document line item with a foreign country

"Please enter VAT rate for non-text line items. [US18000001] (or "You must select a VAT line for non-text items. [US18000001]")."

This error is likely caused by not specifying a VAT line for the country "code:XX" on an issued invoice.
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You can find the VAT lines for invoices under the "clenDph" element here:
​https://demo.flexibee.eu/c/demo/faktura-vydana-polozka/properties
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You can find the individual predefined VAT lines in Flexi here:
​https://demo.flexibee.eu/c/demo/cleneni-dph.xml
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For foreign countries, a VAT line must be specified, for example: "clenDph":18, (in this case, ID 18 corresponds to VAT lines with no VAT impact).