xsd:boolean element type accept “true” but not “True”. How can I make it accept it?

You cannot.

According to the XML Schema specification, a boolean is true or false. True is not valid:

  3.2.2.1 Lexical representation
  An instance of a datatype that is defined as ·boolean· can have the 
  following legal literals {true, false, 1, 0}. 

  3.2.2.2 Canonical representation
  The canonical representation for boolean is the set of 
  literals {true, false}. 

If the tool you are using truly validates against the XML Schema standard, then you cannot convince it to accept True for a boolean.