Answer
The DBMS_REDEFINITION error message is usually secondary to the real problem: insufficient space in the LOB tablespace (commonly IFS_LOB) to actually perform the shrink/redefinition operation — check the fuller error log for an ORA-01691 ('unable to extend lob segment... in tablespace...') alongside it, which confirms this. The fix is to have a DBA increase that tablespace's size (100GB is a reasonable starting point, more depending on your actual LOB data volume) rather than troubleshooting the DBMS_REDEFINITION reference itself. Once there's enough tablespace, expect the very first run of the shrink job to take a long time (potentially a day or two for a large volume of data), so schedule it for genuinely non-peak hours, and running it weekly or monthly (depending on data growth) rather than more frequently is usually sufficient going forward.