Issue creating install script for native unity game - Unable to load mono library from .../libmono.so

I am a fairly experienced Linux user (currently running Pop!_OS) who is new to Lutris, and I’m trying to create an install script for DELTATRAVELER. Extracting the archive manually works, but if I do it with the install script, it fails with the following error message:

Unable to load mono library from /home/eliminmax/Games/deltatraveler-install-test/DELTATRAVELER_Data/Mono/x86_64/libmono.so

The draft install script is as follows:

files:
- game_archive: N/A:Please select the zip file from rynogg.itch.io/deltatraveler
game:
  exe: $GAMEDIR/DELTATRAVELER
installer:
- extract:
    dst: $CACHE
    file: game_archive
    format: zip
- input_menu:
    description: 'Please choose which version to run:'
    id: ARCH
    options:
    - x86_64: x86_64 (64 bit, default)
    - x86: x86 (32 bit)
    preselect: x86_64
- move:
    dst: $GAMEDIR/DELTATRAVELER
    src: $CACHE/DELTATRAVELER.$INPUT
- move:
    dst: $GAMEDIR/DELTATRAVELER_DATA
    src: $CACHE/DELTATRAVELER_Data
- chmodx: $GAMEDIR/DELTATRAVELER

stdout+stderr from lutris -d