Since it came out while developing an application with rails, I will write down the remedy I took. By the way, I encountered it when I tried to implement the posting function with the new action.
new is missing a templete for this request format and variant. The request format of the new template is wrong ...? When I made a low-level translation, I noticed that there was something to read ahead. The following error statement request.formats: ["text / html"] seems to mean that the request is html and that html is (not) wrong.
When I looked at new.html.erb, the file name was typo as new.hml.erb. It was a terrible mistake, but when I fixed it, it worked fine.
If you get an ActionController :: UnknownFormat error, it's okay to check if the file name of the view is a typo, if the view is in the correct location, or if you created the view in the first place.
I hope you find it useful. Thank you for watching until the end.