Ini_set has been disable serverwide. If you need to enable ini_set in your site, you need to tweak php.ini file. You will know by seeing this: "Warning: ini_set() has been disabled for security reasons ..."
If you do not have a copy of the php.ini file in your site, you may download from here. Once you have a copy of the php.ini file, check if the commandline "disable_functions = " is like this:
disable_functions = show_source, shell_exec, passthru, exec, popen, proc_open, allow_url_fopen, ini_set
Delete "ini_set" from the line and save; then upload the file in the directories where the scrips are being called.
For Joomla template, you need to have php.ini files in the home directory and administrator folder. In case you see the error during the installation, include the php.ini also in the /installation folder.