Hi,
Thank you for posting in Symantec community.
I would be glad to answer your query.
Q. Can i upgrade directly from 11.0.6005 to 12.1.5 or not?
--> Yes, it's fully supported. Upgrade asap to stay protected.
To know the best practice I would recommend to go through the following article.
https://www-secure.symantec.com/connect/articles/sep-11x-sep-121-upgrade-process-graphical-overview
Important note: The SEP 12.1 SEPM upgrade installer requires three times the current database size (sem5.db) if upgrading from pre-12.1 SEPM with the embedded database only.
Also, check this article as well.