As of this date, it has been some time since Postleaf has had an update. It will not run on a version of NodeJS higher than 7, which is now deprecated. Higher versions of NodeJS (and npm) will cause dependency issues. Making Postleaf run under NodeJS 10 requires just a few tweaks.
The packages 'gulp-sass' and 'sqlite3' require an update. Update package.json with these version numbers.
After that, delete your node_modules folder and run 'npm install'. It should complete without errors under NodeJS 10.
This change will not fix multiple vulnerabilities, and 'npm audit fix' will not resolve the bulk of issues. Running the command with '--force' will prevent Postleaf from properly starting.