This is the last message in the message stack. It shows where a problem occurs in PL/SQL code. Following this message should be another message something like this:

ORA-06512 at XXXX
ORA-YYYY: error ZZZZ

The messages following the 06512 tell you what (YYYY and ZZZZ) and where (XXXX) the error occurs.

The typical structure of the message is:

ORA-06512 at str line num.