I have a 2017 iMac with no operating system

If you do not have a bootable system of any sort, you may be in for some difficulty trying to get an OS back onto that machine again.

Mac startup key combinations - Apple Support

"Command (⌘)-R: Start up from the built-in macOS Recovery system. Or use Option-Command-R or Shift-Option-Command-R to start up from macOS Recovery over the internet. macOS Recovery installs different versions of macOS, depending on the key combination you use."

Looks like you will need to try Internet Recovery and pray that it still works for a machine that old.

A 27" 2017 iMac would have shipped with macOS 10.12.4 or 10.12.5 (Sierra), so if that's the operating system you install using Internet Recovery, you might need to do a few more installations to get back to Ventura. E.g., a High Sierra installation followed by a Ventura installation.

Update macOS on Mac - Apple Support

A good rule of thumb when considering a Scorched Earth Method 

Is to Always have the Installation Media already in hand versus afterwards

To perform this action will require booting from a Bootable Installer. 

The Bootable Installer can Only be performed on an Apple Computer 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer.

Example : Bootable Installer of Ventura would have to be done on a computer that Qualifies to run Ventura

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can’t be used.

If all avenues have been blocked

If you have the means and time to transport the computer to an Apple Genius Bar, they maybe able to reload macOS Ventura

What macOS does the Recovery suggest (post a screenshot if necessary)? And what Recovery method did you use (Command-R, Option-Command-R, Shift-Option-Command-R)? And what format did you erase the internal disk (APFS, GUID partition scheme, case insensitive must be used for Mojave and later)?

What macOS was used previously? High Sierra introduced APFS so you might have to install that so that the firmware is updated so you can erase as APFS.

What kind of internal disk does that Mac have (SSD, Fusion Drive, HDD)? Maybe the internal drive is broken and must be replaced or you might use an external drive?

How to reinstall macOS - Apple Support

"If the installer doesn't see your disk, your disk might have an issue that requires you to use Disk Utility to erase it. If Disk Utility can’t see it or erase it, your Mac might need service."

This seems to be a know problem in Sierra Recovery so try this:

https://mrmacintosh.com/fixed-an-error-occurred-while-preparing-the-installation-macos-sierra-recovery-error/

...and if you happen to try High Sierra there is also an old Recovery problem and a workaround:

https://mrmacintosh.com/how-to-fix-the-recovery-server-could-not-be-contacted-error-high-sierra-recovery-is-still-online-but-broken/

monokia wrote:

Thanks for your answer.

You are welcome

In this case my iMac wasn’t in the state I could’ve made a bootable usb. So I didn’t have the chance to make it. And I wasn’t aware about that my new laptop can’t download an older mac os.

So now you know