"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.
"average strength" is not in the list (extremely weak, very delicate, quite fragile, rather flimsy, particularly weak, somewhat unsound, appreciably susceptible, marginally vulnerable, average construction, a bit safeguarded, ...) of allowed values for the "Construction is" property.