试一下，把INTO _T 修改成 BULK COLLECT INTO _T
另外，最后要想着 RETURN _T
我测试的过程如下

CREATE SCHEMA SA;

CREATE TYPE SA.StrToColTable IS TABLE OF VARCHAR2(32767);

CREATE OR REPLACE FUNCTION SA.StrToCol(_str TEXT,_separator varchar2(10))
RETURN SA.StrToColTable
IS
  _T SA.StrToColTable;
BEGIN
  SELECT REGEXP_SUBSTR(_str,'[^,]+',1,LEVEL)
    BULK COLLECT INTO _T
    FROM DUAL
 CONNECT BY LEVEL <= REGEXP_COUNT(_str,'[^,]+');
  RETURN _T;
END;

select *
  from table(SA.StrToCol('11,22,33,44,55',','));