Unable to reinstall MacOS on my iMac

This is an ongoing problem with Apple's older macOS versions lately.

Make sure your Mac has a solid wi-fi signal with decent speed, or connect to your router with an ethernet cable. The wi-fi connection can be made by clicking the icon in the upper right corner of the screen when started up in Recovery mode.

Does your Mac startup properly now?

If so, you might create a bootable usb installer. Otherwise, you'll need to use another Mac of similar vintage - one that can run macOS 10.13 High Sierra - to create the installer.

Create a bootable installer - Apple Support

What is the exact model of this Mac? You can get this information by clicking the Apple menu and selecting "About This Mac", or if the Mac cannot be booted to the internal drive, then entering the system serial number here:

Check Your Service and Support Coverage - Apple Support

What version of macOS installer is available when you boot into Internet Recovery Mode using Command + Option + R?

Make sure the system date & time are correct. From recent reports it appears some Macs are now able to access the recovery servers, but the date & time must be correct. Previously it was necessary to configure the date & time to 2017.