It's caused by limitations of the QR code. It's possible to use it just in case of German speaking countries, like Germany, Switzerland and few others. It's not mentioned in the docs, thought.

In that case it should also be available in Slovenia, we also use QR codes for invoices. Or is this limitation hardcoded in odoo just for these few Countries?

We are using QR codes as well in the czech Republic. But it's country specific. I asked the Odoo support and they told me, that :

"The EPC QR Code feature is only available for companies in several European countries such as Austria, Belgium, Finland, Germany, and the Netherlands."

Sorry, I wrongly remembered Switzerland. You have to reimplement this functionality for your country yourself. I am working on our implementation. Biggest problem is to reimplement view from scratch, as I don't like current Invoice structure, also it's missing some necessary parameters, that are required by Czech tax law.

Ok thank you for your feedback. Did you by any chance find out how to change "payment reference" code? Odoo is using "RF" prefix for references, but in Slovenia shops usually use "SI" prefix, and I can't find where this could be changed inside Odoo.

Unfortunately this doesn't work. We follow exactly those steps and in the Other info tab in the Invoice, we see just empty "Payment QR-code". Only option is to choose "SEPA Credit transfer QR", but this doesn't work with our currency and generation of invoice fails.