See image-upload branch

your design do crazy stuff @falko20
show.erb.txt

there is no show stuff in app/views/admin/events/show.html.erb

selections look like this:
<%= form.select :country, @countries, class: "form-control custom-select", :placeholder => "Choose the country", :require => true %>
with this solutions html tags (e.g. class) dont work
select takes the following arguments select(method, choices, options="", htmloptions="")
therefore it should look like this:
<%= form.select :country, @countries, {}, {class: "form-control custom-select", :placeholder => "Choose the country", :require => true} %>

please consider this while creating forms and update the existing ones

ActionController::RoutingError (No route matches [GET] "/admin/sb-admin.js"):
change import js in html head or fix it

frontend/events/1

Faker for database events.description isn't a quote but "Faker::Quotes"

for example here