Hi
the template is not officially ready for Joomla3.6.4. The updates are under process on priority basis .
If you want to update site I suggest you test site on localhost first and check if you do not have any issue and do not play with the live site .
for above issue is not due to the template ,its due to the extension tcpdf not ready for Joomla 3.6.4
For the error, a user solved it with reinstalling pdf .
Uninstall com_tcpdf via Extensions > Manage > Manage
Install com_tcpdf_1.0.2.zip
You can get it from http://dev.virtuemart.net/attachments/download/1022/com_virtuemart.3.0.18_extract_first.zip
Always take backup of site before updating Joomla .