Hi BaoTran94,
64 bit support is required for After Effects CS6.
Please make sure your computer meets the minimum system requirements : http://www.adobe.com/in/products/aftereffects/tech-specs.html
You may also want to download using the direct download links available at : http://prodesigntools.com/adobe-cs6-direct-download-links.html
But make sure you follow the important instructions on the page before initiating the download.