"Processing error message" is a predefined property containing a textual description of an error and is provided by Semantic MediaWiki. This property is pre-deployed (also known as special property) and comes with additional administrative privileges but can be used just like any other user-defined property.
S
Property "Modification date" has a restricted application area and cannot be used as annotation property by a user.
W
Some use of "[[" in your query was not closed by a matching "]]".