Hi,
For some reason your OS can’t see your hard drive. This could be because the system is corrupted, the hard drive failed or your HD is encrypted.
FIRST OF ALL, do you have any backup at all? As a rule of thumb the important stuff has to always be backed up either to some cloud service or external disk.
IF YOU HAVE NO BACKUP, consider if you are willing to erase and lose data, otherwise you might want to look for some data recovery company to try and salvage your data.
Try this:
• Make sure you have WiFi or Ethernet connection available for the next step.
• Start on recovery mode with command + option + R when turning your Mac on.
• Select disk utility. You should have an option to View - All devices or something similar on your menu.
• Can you see any hard drive there? Is it greyed out? If so, try to Mount the greyed out disk from the top menu. If your disk is encrypted you’ll need the encryption password (this is user generated, either you or a previous user needs to have the password as Apple cannot retrieve it).
• Try running First Aid on all disks and see if anything is found. The system will try to correct potential file structure errors.
• IF YOU ARE WILLING TO ERASE DATA, can you select the uppermost drive to the left (once you select View all devices)? If so, select that disk and erase it.
• After erasing it, you can quit disk utility and attempt installing macOS again.
Hope this helps.
Vicko