'jekyll pagemaster' could not be found.

I have the same issue as #17 Running bundle exec jekyll pagemaster monsters results in:

fatal: 'jekyll pagemaster' could not be found. You may need to install the jekyll-pagemaster gem or a related gem to be able to use this subcommand.

my Gemfile:

source "https://rubygems.org"
group :jekyll_plugins do
    gem 'github-pages'
    gem 'jekyll-admin'
    gem 'pagemaster'
end

My _config.yml:

staticman:
  branch: gh-pages
permalink: /:slug-:month:day:short_year
include: ['_pages']

title: Jekyll-db
description:
google_analytics:
show_downloads: true
plugins:
  - pagemaster

collections:
    monsters:
    output: true
    source: monsters.csv
    id_key: id
    layout: entry

I have:

* pagemaster (2.0.2)
    Summary: jekyll pagemaster plugin
    Homepage: https://github.com/mnyrop/pagemaster
    Path: /home/yochai/gems/gems/pagemaster-2.0.2

Bundle info:

Using concurrent-ruby 1.1.7
Using i18n 0.9.5
Using minitest 5.14.1
Using thread_safe 0.3.6
Using tzinfo 1.2.7
Using zeitwerk 2.4.0
Using activesupport 6.0.3.2
Using public_suffix 3.1.1
Using addressable 2.7.0
Using backports 3.18.2
Using bundler 2.1.4
Using coffee-script-source 1.11.1
Using execjs 2.7.0
Using coffee-script 2.4.1
Using colorator 1.1.0
Using ruby-enum 0.8.0
Using commonmarker 0.17.13
Using unf_ext 0.0.7.7
Using unf 0.1.4
Using simpleidn 0.1.1
Using dnsruby 1.61.4
Using eventmachine 1.2.7
Using http_parser.rb 0.6.0
Using em-websocket 0.5.1
Using ffi 1.13.1
Using ethon 0.12.0
Using multipart-post 2.1.1
Using faraday 1.0.1
Using forwardable-extended 2.6.0
Using gemoji 3.0.1
Using sawyer 0.8.2
Using octokit 4.18.0
Using typhoeus 1.4.0
Using github-pages-health-check 1.16.1
Using rb-fsevent 0.10.4
Using rb-inotify 0.10.1
Using sass-listen 4.0.0
Using sass 3.7.4
Using jekyll-sass-converter 1.5.2
Using listen 3.2.1
Using jekyll-watch 2.2.1
Using rexml 3.2.4
Using kramdown 2.3.0
Using liquid 4.0.3
Using mercenary 0.3.6
Using pathutil 0.16.2
Using rouge 3.19.0
Using safe_yaml 1.0.5
Using jekyll 3.9.0
Using jekyll-avatar 0.7.0
Using jekyll-coffeescript 1.1.1
Using jekyll-commonmark 1.3.1
Using jekyll-commonmark-ghpages 0.1.6
Using jekyll-default-layout 0.1.4
Using jekyll-feed 0.13.0
Using jekyll-gist 1.5.0
Using jekyll-github-metadata 2.13.0
Using mini_portile2 2.4.0
Using nokogiri 1.10.10
Using html-pipeline 2.14.0
Using jekyll-mentions 1.5.1
Using jekyll-optional-front-matter 0.3.2
Using jekyll-paginate 1.1.0
Using jekyll-readme-index 0.3.0
Using jekyll-redirect-from 0.15.0
Using jekyll-relative-links 0.6.1
Using rubyzip 2.3.0
Using jekyll-remote-theme 0.4.1
Using jekyll-seo-tag 2.6.1
Using jekyll-sitemap 1.4.0
Using jekyll-swiss 1.0.0
Using jekyll-theme-architect 0.1.1
Using jekyll-theme-cayman 0.1.1
Using jekyll-theme-dinky 0.1.1
Using jekyll-theme-hacker 0.1.1
Using jekyll-theme-leap-day 0.1.1
Using jekyll-theme-merlot 0.1.1
Using jekyll-theme-midnight 0.1.1
Using jekyll-theme-minimal 0.1.1
Using jekyll-theme-modernist 0.1.1
Using jekyll-theme-primer 0.5.4
Using jekyll-theme-slate 0.1.1
Using jekyll-theme-tactile 0.1.1
Using jekyll-theme-time-machine 0.1.1
Using jekyll-titles-from-headings 0.5.3
Using jemoji 0.11.1
Using kramdown-parser-gfm 1.1.0
Using minima 2.5.1
Using unicode-display_width 1.7.0
Using terminal-table 1.8.0
Using github-pages 207
Using rack 1.6.13
Using rack-protection 1.5.5
Using tilt 2.0.10
Using sinatra 1.4.8
Using multi_json 1.15.0
Using rack-test 1.1.0
Using sinatra-contrib 1.4.7
Using jekyll-admin 0.10.2
Using pagemaster 2.0.2

mnyrop / pagemaster

'jekyll pagemaster' could not be found. #29