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).