The attribute is not allowed

An attribute the schema does not declare for this element.

The message, as the validator reports it

Element 'order', attribute 'vat': The attribute 'vat' is not allowed.

Why it happens

An attribute from a newer or different version of the format.
A misspelt attribute name.
An attribute that belongs to a child element, not this one.

How to fix it

Remove the attribute or rename it to one the schema declares. xmlns and xsi:schemaLocation are always allowed; anything else must be declared.

Example

Validated against order.xsd, below.

Reports the error

<order currency="EUR" vat="20">
  <id>A-1</id>
  <qty>1</qty>
</order>

Fixed

<order currency="EUR">
  <id>A-1</id>
  <qty>1</qty>
</order>

order.xsd

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="order">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="id" type="xs:string"/>
        <xs:element name="qty" type="xs:int"/>
        <xs:element name="status" type="Status" minOccurs="0"/>
        <xs:element name="code" type="Code" minOccurs="0"/>
        <xs:element name="note" type="Short" minOccurs="0"/>
      </xs:sequence>
      <xs:attribute name="currency" type="xs:string" use="required"/>
    </xs:complexType>
  </xs:element>
  <xs:simpleType name="Status">
    <xs:restriction base="xs:string">
      <xs:enumeration value="open"/>
      <xs:enumeration value="paid"/>
    </xs:restriction>
  </xs:simpleType>
  <xs:simpleType name="Code">
    <xs:restriction base="xs:string">
      <xs:pattern value="[A-Z]{3}-[0-9]{4}"/>
    </xs:restriction>
  </xs:simpleType>
  <xs:simpleType name="Short">
    <xs:restriction base="xs:string">
      <xs:maxLength value="10"/>
    </xs:restriction>
  </xs:simpleType>
</xs:schema>