This error message indicates that one of your metadata nicknames has identical value as noted in examples of the error message. To correct this you need to rename the nickname that carries the reserved syntax. Typically, this is "data", which most sites tend to assign with secondary date field.
To access your nickname editing tool, open CONTENTdm administration, go to Items > Index and select your collection (change into it). Replace "bld.exe" part of your URL with "convertnickname.exe". Only replace the bld.exe, leave the rest of the url after this term intact, then hit Enter.
After you update your nickname, re-index the collection, then close the project and re-open it in Project client to refresh the collection settings.
There is more on using the Nickname tool.