This error happens if you don't redirect in the
create method of your controller.
Are you redirecting in the create method in the controller or rendering the new form, in case of error?
Without the redirection in the create method in the controller, you need to make a new file called
create.html.erb. Usually, after successful creation, you redirect to some other page like shown below
# some object you want to create
# if the object.save is fine
# redirect_to object
# render new with the errors