PROBLEM:

While enabling parallel DML option, after doing some transaction, got error ORA-12841

SOLUTION:

If we are enabling parallel DML option, after doing some transaction, then we need to issue COMMIT or ROLLBACK, before enabling DML parallel.

oerr ora 12841
12841, 00000, “Cannot alter the session parallel DML state within a transaction”
// *Cause: Transaction in progress
// *Action: Commit or rollback transaction and then re-execute

So just issue COMMIT and try to run alter session enable parallel command.