ORA-01027:variáveis ​​de ligação não permitidas para definição de dados ao tentar usar if elseif

create or replace procedure create_dates_testing 
    ( dummy_variable varchar2 default to_char(sysdate,'YYYYMMDD') )
as
    day_of_month varchar2(255) := extract(day from sysdate);
    today varchar2(255) := to_char(sysdate +1, 'fmDAY', 'nls_date_language = English');
    start_date date;
    next_start_date date;
begin
    if today = 'SUNDAY' then
        -- select yesterday
        start_date      := trunc(sysdate) - interval '1' day;
        next_start_date := trunc(sysdate);

    elsif day_of_month = 3 then
        -- select the whole of last month
        start_date      := trunc(sysdate, 'MM') - interval '1' month;
        next_start_date := trunc(sysdate, 'MM') - interval '1' month;
    else
        return;
    end if;

    execute immediate 'drop table new_customers';

    execute immediate 'create table new_customers as 
    select id, client_name, invoice_date 
    from clients table
    where transactiondate >= date '''  || to_char(start_date,'YYYY-MM-DD') ||
    ''' and transactiondate < date ''' || to_char(next_start_date,'YYYY-MM-DD') ||'''';

end create_dates_testing;