The attribute is required but missing

An attribute declared with use="required" is not on the element.

The message, as the validator reports it

Element 'order': The attribute 'currency' is required but missing.

Why it happens

The attribute was left out, or misspelt.
A prefixed attribute where the schema expects an unprefixed one (or the reverse): attributes are in no namespace unless they have a prefix.

How to fix it

Add the attribute with a valid value.

Example

Validated against order.xsd, below.

Reports the error

<order>
  <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>