In order to issue an invoice to a legal or natural person with identification, you need to create it as a client in your cash register in the Settings / Company / Partner option. Then on the blue circle with a white plus sign in the lower right corner, after which a window for entering client data opens.

You must indicate that it is a client. The first two fields are mandatory. The TIN field is the field where you enter,

for a legal entity, mark 10: then the user's PIB (eg: 10:112113458), for a natural person 20: then the user's ID card number (eg: 20:011234545). The content in this field will appear on the invoice, which as such will be an invoice that meets the legal content for issuing invoices with customer identification.