You probably wiped out the recovery partition when installing ChromeOS. To solve this, you'll likely need to create a MacOS installer flash drive, boot the computer holding the "option" key, then boot from said USB disk. Then you can reformat the hard drive and install MacOS.
You can download USB installer images for older Macs [link|https://support.apple.com/en-us/102662|directly from Apple here.|new_window=true] The newest version your machine can run is likely High Sierra but only a download of Sierra is available. Just install that then upgrade if desired.
To make the bootable USB installer, reference [link|https://support.apple.com/en-us/101578|this guide|new_window=true]. If you don't have access to another Mac, there are ways to create an installer on a PC as well. It should be relatively easy to find with some googling.