This article explains the BACKTRACK error message “Main database set-up has failed with error code (-15)” and details how to fix the underlying problem that caused it to appear.
BACKTRACK is unable to create a new database during install. There could be multiple causes of this. In this case, it appears the version of SQL (i.e. 2008) had a conflict with BACKTRACK v6.
If you are using SQL 2008, upgrade to SQL 2008 R2 to resolve the issue. The upgrade of SQL fixed the issue with BT6 installed on XP.
Error message first appeared: Windows XP running BACKTRACK v6