If you use postfix with DKIM milter and start getting such error:

no key selected for signing 
milter-reject: END-OF-MESSAGE from localhost

make sure you have correct version of milter_protocol in your main.cf:

/etc/postfix/main.cf:
 # Postfix ≥ 2.6
 milter_protocol = 6
 # 2.3 ≤ Postfix ≤ 2.5
 milter_protocol = 2