I bought new Lenovo all in one pc without os and when i boot it up i only see white screen?

You bought a pile of electronics, that are Worthless without an Operating System.

Yeah you are going to need to buy an OS.

You can use a keyboard access to the BIOS screen. That is all you will see until you try to install an operating system. If you can't see the BIOS entries, something is damaged.
That is all that can be replied from "new Lenovo all in one pc without os" and "see white screen" without a model number and a place of purchase.

Depending upon what model it is and how new, you need to web search for the key that will get you into BIOS or UEFI. Then assuming that you have another computer that you could use to put Linux on DVD or create a bootable USB, to boot/install Linux on its hard drive, assuming that it has a drive. Or you could buy Windows.

I'm sorry. Nah, not really. It's hard to feel sorry for somebody that buys a POC all in one. ESPECIALLY one without an OS!

Are you sure? Maybe it has a recovery partition on it. Lenovo's all have one-key recovery, tap F12 at system start, or try tap F11 at system start. Both are only valid for a few seconds after power on.

Ok let check lenovo

Who sells new computers without an OS? The Levano series of AIO units all come with Windows 10 pre-installed. The only way you could avoid that would be to have one custom built for you. No computer can do more than self test and show that it works without an OS. If you custom ordered it without an OS, you now get to buy your own Windows OS at a computer store. (Or using another computer, downloading a free version of Linux to install.)