create or replace procedure p_createseq(tablename in varchar2) is
   /*错误位置在case需要改成cache 这个是正确的 */
  strsql varchar2(500);
  a      NUMBER(4) := 0;
begin
  select max(ec_id) into a from easybuy_comment;
  strsql := 'create sequence seq_' || tablename ||
            ' minvalue 1 maxvalue 9999 start with '||a||'
            increment by 1 cache 20'; 
  execute immediate strsql;
end p_createseq;

------解决方案--------------------
declare
 a     int;
 v_sql varchar2(500);
begin
 select max(id) into a from sys_user;
 --ddl语句不能直接在plsql里执行，要通过动态语句执行
 v_sql := 'create sequence commentid minvalue 1 maxvalue 9999 start with ' || a ||
         ' increment by 1 cache 20'; --3楼这里写错了 是cache不是case 这个语句应该是没错的
 execute immediate v_sql;
end;