Olá,
Tente estas soluções
http://stackoverflow.com/questions/7530725/unrecognized-attribute-targetframework-note-that-attribute-names-are-case-sen
http://stackoverflow.com/questions/17510440/unrecognized-attribute-targetframework-note-that-attribute-names-are-case-s
http://stackoverflow.com/questions/10630473/unrecognized-attribute-targetframework-note-that-attribute-names-are-case-sen