Startup disk

If you have a backup on an external drive you can connect it and choose that as the startup disk.

Otherwise, you can start in Recovery mode. About macOS Recovery - Apple Support

From there you can install macOS.

jip128 wrote:

I just deleted my hard drive and now I want to restart my mac. To restart I have to choose a startup disk, but there are no options. How do I get the startup disk in the options? Or how can I restart my mac?

Hopefully this accomplished what you were hoping for; if you have a bigger issue that lead to this, then explain what that issue is, otherwise everything you need is here:

How to erase a disk for Mac - https://support.apple.com/en-us/HT208496

How to reinstall macOS from macOS Recovery - https://support.apple.com/en-us/HT204904

OK. Try to explain why you deleted your hard drive. Is it a rotating drive or solid state? What format did you choose when you erased it? What year is your Mac and what was the operating system when you erased the drive? Did you run First Aid as suggested?

Also, power down the computer and hold down the Option key on powering back on.

So you do not have a backup?

Use the Utilities menu in Recovery mode, choose Disk Utility and erase your hard drive again from there.

Then use the Recovery menu to install macOS.

What message do you get when you try?

In Recovery mode / Utilities / Disk Utility select your hard drive and run First Aid.