My Mac Pro won't boot up after wiping

I have an old intel based Mac Pro (late 2013) that I plan to sell. I wiped my internal SSD but I think I messed up. I booted it up in recovery mode, erased the drive using Disk Utility and then clicked restart. But I guess I should have reinstalled the OS first. Now when I try to start it I get a question mark. Trying to re-boot it in recovery mode hasn’t worked. I just keep getting the question mark. Any advice?

[Re-Titled by Moderator]