imac repair

It would help to have more details on exactly what happens and any specific error messages.

It would also help to know the exact model of the Mac which you can get by clicking the Apple menu and selecting "About This Mac" or by entering your system serial number here:

Check Your Service and Support Coverage - Apple Support

Also knowing why you are attempting to reinstall macOS may be pertinent as the problem you are trying to resolve with a reinstall of macOS may be the same problem that is causing the reinstall to fail.

Writing an effective Apple Support Community - Apple Community

Click ▶︎ Reinstall macOS.

If that didn’t work, then I recommend service.

Service Checklist:

• Mac service pricing.
• Check AppleCare Coverage.
• Get your Mac ready for service.

Click ▶︎ Start a Service Request. ⚛️

Need more help?

• Call Apple Support at 1-800-APLCARE. 📞
• International Support. 🌐
• Contact Apple Support. 📝
• Chat using Messages. 💬

Click ▶︎ Check Repair Status. 🔧