Sorry, this is a deep onion problem which really will need reformatting and rebuilding to fix. Hopefully you have a backup of your stuff.
Your other option which you might want to look at is to get a bigger drive, leaving this drive alone for now and then using a special case and a data recover app try to recover your stuff.
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=== Update (11/08/2021) ===
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Using a Mac download from here the Sierra installer [link|https://support.apple.com/en-us/HT211683c|How to get old versions of macOS]
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The drive you are using needs to be formatted with GUID and a journaled file system to work properly. So you will likely need to reformat it using your Mac (or someones else’s).
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With that done copy over the installer and then launch it to unpack it and drag out if needed so the it can be run on its own, exit if you launch it.
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Now we need to convert it into a self booting installer following this guide [link|https://www.macworld.com/article/228350/how-to-create-a-bootable-macos-sierra-installer-drive.html|How to create a bootable macOS Sierra installer drive]
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Once done reboot your system with this drive connected. It should latch on its own if not use the Boot Manager, restarting your system again and this time press the ***Option (⌥)*** key and select your installer drive using the arrow keys and enter.
Ouch!
Sorry, this is a deep onion problem which really will need reformatting and rebuilding to fix. Hopefully you have a backup of your stuff.
Your other option which you might want to look at is to get a bigger drive, leaving this drive alone for now and then using a special case and a data recover app try to recover your stuff.