ORACLE/PLSQL: ORA-04091

You are using a function in an insert statement which is reading data from the table it is trying to change.

Relational DBs have to be atomic (anything a DML statement changes is assumed to happen at the same time) and as such things that act on data in DML have to be deterministic - i.e. give the same result even if execution plan and concurrency differs.   Your situation isn't deterministic, so Oracle throws an error.

The easiest solution is to select the result of taksit_bul (p_national_id) into a variable, and use the variable in the insert statement.  That should cure the mutating table.

v_taksit_bul := taksit_bul (p_national_id);

[snip]....

         INSERT INTO ps_acu_sf_tahs

            SELECT 'ACU',

                   b.emplid,

                   b.acad_career,

                   b.stdnt_car_nbr,

                   b.acad_prog,

                   ps_acu_tahs_sequence.NEXTVAL,

                   TO_DATE (a.updated, 'RRRRMMDD'),

                   v_acu_debit_term,

                   v_acu_debit_type,

                   v_acu_payment_term,

                  v_taksit_bul, -- NEW CHANGE HERE!!!

                   v_acu_payment_typ,

                   a.amount,

                   v_currency_cd,

                   v_acu_kur,

                   a.amount * v_acu_kur,

                   'TUR',

                   v_bank_cd,

                   v_branch_ec_cd,

                   v_account_num,

                   'N',

                   v_oprid,

                   SYSDATE,

                   v_comments

              FROM ps_acu_tahs a, ps_acu_stu_info b

             WHERE     a.national_id = b.national_id

                   AND a.national_id = p_national_id

                   AND acu_flag3 = p_flag

                   AND prog_status IN ('AC', 'LA');