Memory error after confirm check out in virtuemart

Hi i have encounter memory error below after i confirm my order
Can you advice what is happening and what need to be change?
Fatal error: Allowed memory size of 33554432 bytes exhausted (tried to allocate 491520 bytes) in /home/doterra/public_html/libraries/tcpdf/tcpdf.php on line 20900

follow that file path to tcppdf.php and after <?php at start of file insert this:

ini_set('memory_limit', '-1');

Hi, i tried

<?php ini_set('memory_limit', '-1');
//============================================================+
// File name : tcpdf.php
// Version : 5.9.146
// Begin : 2002-08-03

also

<?php
ini_set('memory_limit', '-1');
//============================================================+
// File name : tcpdf.php
// Version : 5.9.146
// Begin : 2002-08-03
// Last Update : 2012-02-12

It's still getting same error message

Fatal error: Allowed memory size of 33554432 bytes exhausted (tried to allocate 491520 bytes) in /home/doterra/public_html/libraries/tcpdf/tcpdf.php on line 20900

See this , exact problem as yours:
forum.virtuemart.net/index.php?topic=110384.0

you will have to ask provider to increase memory limit for you.

Thank you,
I had created a php.ini file in webroot directory
include the following in the php.ini file
memory_limit = 128M;
max_execution_time = 300;
value upload_max_filesize 12M;

After this my problem solved