The user created their tables in the SYSTEM schema and when they try to use Convert Schema or Migrate data for the SYSTEM schema the options are greyed out.
SSMA does not migrate system objects
SSMA considers all objects in SYS and SYSTEM schemas as system objects. SSMA does not migrate objects which is considered to be system objects of Oracle in SSMA intentionally. Creating objects in another schema and do the migration.
Article Id: KBO2SS1010
SSMA 2005 for Oracle
SSMA 2008 for Oracle