I usually manage my node with nvm - I avoid installing npm packages using sudo.
I got the same error - you are missing the gulp package.
npm install -g gulp
Cannot package the path '/home/fh/pkgbuilds/mullvad-vpn/src/mullvadvpn-app/dist-assets/mullvad', does it exist?", :level=>:error}
The error messages on my system indicates the script is trying to build a deb package.
There seems to be missing a build sequence somewhere in the PKGBUID because files are missing which the
npm run pack:linux looks for.
If you look at the yaml file in the dist folder - there is something missing.