If you received a "OOOOPS, THERE'S AN ERROR HERE." error when you attempted to run a report in Simple Invoices this means that your version of PHP doesnt have the correct extentions installed(or enabled).
If your running a Windiws server and using WAMP5 please refer to the page on the Simple Invoices wiki for information on how to fix this https://mirror2.cvsdude.com/trac/simpleinvoices/simpleinvoices/wiki/wamp5_reports
If your running Unix/Linux and PHP5 please make sure you have the xsl extension installed and enabled in your php.ini. On Ubuntu GNU/Linux please install the php-xsl package for PHP5
If your using PHP4 please make sure that your PHP has Sablotron support "--enable-xslt"
Note: If you are using Simple Invoices in the demo environment on SourceForge reports wont work due to setup of their servers