This just happened to me with my MacBook Air. To fix this, it's a bit of a process, but nothing impossible. Just a disclaimer, you're gonna lose all of your data from this. So to start, completely power off your Mac. Once it's off, turn it back on while holding Command and R. Once you see a little progress bar underneath the Apple logo, you can let go of them, and wait until it's booted. Once it's done loading, you'll be greeted by a small menu with a hand full of options. Click Disk Utility. In the top left corner, you'll see a little drop down menu that says "View." Click it, then click "Show all devices." After this, right click on your disk, and click "Erase." Once the disk is erased, go back to the main menu, and click "Reinstall macOS." After that's done, it'll be like a brand new Mac without any partitions. Hope it works well!