Permission denied error

[geotheory]

I'm having difficulty getting get this working. My full (reproducible) configuration is here, but the key bits are:

pelicanconf.py

# GALLERY_OUTPUT_PATH variables were not already defined (as implied in docs)
BASE_PATH = '/'
OUTPUT_PATH = 'output/'
GALLERY_FOLDER = 'images/gallery'

#Pelican Conf
PLUGIN_PATHS = ["my_plugins"]
PLUGINS = ["my_plugins.gallery"]

GALLERY_FOLDER = "galleries"
GALLERY_SRC_PATH = "%s%s" % (BASE_PATH, "gallery_src")
GALLERY_OUTPUT_PATH = "%s%s%s" % (BASE_PATH, OUTPUT_PATH, GALLERY_FOLDER)
GALLERY_REGENERATE_EXISTING = False
GALLERY_PRESETS = [
                    {"name": "thumb", "actions": [{"type": "fit", "height": 100, "width": 100, "from": (0.5, 0.5) }]},
                    {"name": "slider", "actions": [{"type": "fit", "height": 300, "width": 900, "from": (0.5, 0.5) }]},
                    {"name": "large", "actions": [{"type": "resize", "height": 640, "width": 850, "from": (0.5, 0.5) }]},
                    {"name": "thumb_greyscale",
                        "actions": [
                            {"type": "fit", "height": 100, "width": 100, "from": (0.5, 0.5) },
                            {"type": "greyscale"}
                        ]},
                  ]

# This setting is optional, used for thumbnails in bootstrap
THUMBNAIL_GALLERY_CLASS = "span2"

This is mostly reproduced in publishconf.py (except PLUGIN_PATHS and PLUGINS). And I have a single page with header:

Title: Visualisations
Slug: visualisation
Gallery: images/gallery

Beyond this I'm unclear what needs to happen. The documentation seems (to me) to leave some bits unspecified. For example I'm unclear what needs to go in the post other than the header. In any case when I try and compile I get:

Robins-MacBook-ProX:pelican_gallery robin$ make publish serve
pelican /Users/robin/Documents/OSU/pelican_gallery/content -o /Users/robin/Documents/OSU/pelican_gallery/output -s /Users/robin/Documents/OSU/pelican_gallery/publishconf.py
ERROR: Could not process pages/visualisations.md
  | PermissionError: [Errno 13] Permission denied: '/output'
Done: Processed 0 articles, 0 drafts, 0 pages and 0 hidden pages in 0.13 seconds.
cd /Users/robin/Documents/OSU/pelican_gallery/output && python3 -m pelican.server

Any assistance much appreciated.