the issue is caused by the script.
look closely at the error message it reports the issue at line 12:
output started at CuraRomana Large official/curaromana/cura/Prep/go/checkbox.php:12
line 12 is an echo statement:
echo "<pre>";
any php script that you use echo in, must be after the <body> tag of the page and should not be included in the head.