Which plugin are you trying to install? I only found this one: https://github.com/jbox-web/redmine_jenkins and it seems to be a little outdated.
In fact, when trying to install it, I get:
bundle install --without development test --no-deployment
The dependency tzinfo-data (>= 0) will be unused by any of the platforms Bundler is installing for. Bundler is installing for ruby but the dependency is only for x8
6-mingw32, x64-mingw32, x86-mswin32. To add those platforms to the bundle, run `bundle lock --add-platform x86-mingw32 x64-mingw32 x86-mswin32`.
Fetching gem metadata from https://rubygems.org/...........
Fetching version metadata from https://rubygems.org/...
Fetching dependency metadata from https://rubygems.org/..
Bundler could not find compatible versions for gem "nokogiri":
In snapshot (Gemfile.lock):
nokogiri (= 1.8.1)
nokogiri (~> 1.8.1)
jenkins_api_client (~> 1.3.0) was resolved to 1.3.0, which depends on
nokogiri (~> 1.6.0)
Running `bundle update` will rebuild your snapshot from scratch, using only
the gems in your Gemfile, which may resolve the conflict.
which means that that version of the Jenkins plugin for Redmine, is not compatible with our latest Redmine stack.
Are you trying to use a different plugin?