The problem is that in Spain, the need of generating a "cash receipt" (ticket de caja) disappeared a long time ago and now it is mandatory to generate a "simplified invoice" (factura simplificada), where several aspects have to be fulfilled: 

1 - The amount of the purchase does not exceed 400 €, VAT included* ❌ Odoo POS allows to make purchases over 400 € with a cash receipt, but this is not legal in Spain.

2 - Number and date of issue, both correlative* ❌ Odoo POS does not use a correlative numbering to the date. 
For example, I add a new order (Ref. 001-001-001) and the customer makes a purchase, then I add a new order (Ref. 001-001-002) and the customer finally does not make the purchase, then I add a new order (Ref. 001-001-003) and the customer makes a purchase. As you can see, the receipt number is not correlative in number to my real sales, there are numbering jumps every time I click on new order, but then it is possible that the sale is not made for any reason.
 The same thing happens with the date, for example, I add a new order (Ref. 001-001-001) and the customer takes a long time to make the purchase, then I add a new order (Ref. 001-001-002) and the customer makes a quick purchase, the date (Ref. 001-001-002) will be before (Ref. 001-001-001-001) and this is not legal in the tax obligations in Spain. For all this to work with Spanish regulations, the assignment of the numbering would have to be done each time the payment is consolidated. 

3 - NIF and identification of the sender ✔️

4 - Identification of the good delivered or services rendered* ✔️

5 - Tax rate (tipo inpositivo) and, optionally also the expression "VAT included" (IVA incluido) . If there are transactions subject to different tax rates must be specified separately and in addition, the part of the Taxable Base (base imponible) corresponding to each transaction* ✔️

6 - Total consideration* ✔️ (Contraprestación total)

Here you can find extra information from the Spanish Agencia Tributaria.