downgrading is a problem

Well… No and YES!

No - You can’t just install Sierra on your drive as the file system was altered when your system was upgraded to High Sierra (or newer). So that’s the issue you are facing. Sierra requires HFS+ and High Sierra was the first version of APFS.

Yes - So how do you get back?

You’ll need to first make a backup of your stuff as you’ll need to scratch your drive down and rebuild it using Sierra.

• Step 1: Get the OS installer from here How to get old versions of macOS
• Step 2: Using Disk Utility format a 32GB USB thumb drive with GUID and a journaled file system (HFS+)
• Step 3: Copy over the installer to the USB and convert it to a bootable installer following this guide How to Make a Bootable macOS Sierra USB Install Drive

Using the Startup Manager Option (⌥) key make sure your USB is bootable. Now you’re set to reformat the drive and install Sierra!

Reference: Mac startup key combinations