I am trying to use validate my xml with UBL 2.1 standards in line with Saudi Arabia ZATCA(Zakat, Tax and Customs Authority) specifications.The url of this validatoe : 
- In the element specifications, find each attribute that its relationship starts with [1 .. this attribute is mandatory, make sure you provide a value for it.

- Check the order of the attribute and make sure they are added in the correct order.

Doing that will ensure that all your elements are compliant to the UBL specifications.
