I am running CUCM 188.8.131.5201-1 and trying to upgrade to the new 7.1(5b) "184.108.40.206000-1".
When trying to pull the upgrade from the SFTP I get the following message "Upgrade candidates found in the patch directory are not allowed by the current version".
7.1(5)SU1 is 220.127.116.1100-6 so you are basically one ES build past that. You'll note that the compatibility matrix indicates you can only upgrade to 7.1(5b) from 7.1(5a) or 7.1(5) base. Michael's blog entry will help you understand why 18.104.22.168000-1 is not actually a higher version than 22.214.171.12401-1.
Some helpful links:
CUCM compatibility matrix
CUCM Upgrade Tool