wysiwyg-rails | Ruby gem for Froala jQuery WYSIWYG HTML Rich Text Editor | Editor library

Your rails code looks fine, but your form is getting double-submitted when you submit. You should be able to see this in the Chrome dev tools as below.

Both of those references point to the application.js file, but different lines. It's hard to tell which libraries are catching the submit because the file is concatenated and doesn't include the debug information. Without seeing which JS files you're using (from application.js), I'd venture a guess that you might have both rails-ujs & jquery_ujs included which is causing the double submit.

Check the related question and answers here: Form submitted twice, due to :remote=>true

Update: You've definitely got both rails-ujs & jquery_ujs present in your compiled JS file. I think that's a good place to look. If you're on Rails 5 and using the jquery-rails gem you might look here: https://github.com/rails/jquery-rails#installation

I found the problem,it's rails version. It's working in rails 5.0.1 now.

I ended up rebuilding the gem file line by line starting with just rails and pg and then pasting and bundle installing each new line.

I found I was hanging on "Sinatra".

None of the errors I found in bundling the complete gem file appeared in the line-by-line bundle. Sigh.

Cant add this as a comment so apologies for the answer which isn't an answer. I upgraded an app to 5.0.2 today and deployed successfully, looking at other questions on the 'don't know how to build' error, it seems to point at the 'require' lines in the capfile. I would suggest you take a look at that & make sure everything is in order.