30% of consumed workload has syntax errors. Check tuning log for more information.
You can see above error (percentage ratio may be changed in case to case) and think that your query is perfect and there is not error at all than why DTA (Database Tuning Advisor) showing syntax error?
Actually DTA is not much powerful to understand if you are using temp table in your stored procedure or calling sub stored procedure or may be using user define functions and it shows above warning. DTA works well only with simple Vanilla type plain SELECT statement.
So in that case, you have to find out the query on which you have doubts, run that query and records it in profiler and get help of DTA.
Reference: Ritesh Shah
Note: Microsoft Books online is a default reference of all articles but examples and explanations prepared by Ritesh Shah, founder of http://www.SQLHub.com