If you install {{VirtualBox}}, like I did last week, [at least] under {{Windows 10}}, and you have not gone into the BIOS ahead of time to enable virtualization, you will be limited to 32-bit guest OSes.
Enable {{virtualization}}, and the 64-bit options become available.
Not sure why you have to enable the hardware virtualization extensions to get {{64-bit}} guest support (nor why it isn’t enabled by default on laptops like the {{Lenovo T450s}} which are aimed at business users), but you do.
Thankfully, you can enable virtualization after the install, and you don’t need to reinstall (which wouldn’t be a huge deal, but certainly an annoyance).
fairly certain this would apply on other vendors’ offerings, too