The plugin is expecting the POST parameter to be sectionId rather than opiniones. Changing the 'name'name attribute on that hidden input to sectionId should fix your issue.
The other requirements when creating entries from front-end template using Guest Entries are outlined below.
Make sure Allow guest submissions? is checked in the plugin’s settings (example.com/admin/settings/plugins/guestentries) and set a default author for the section entries will be saved to.
<input type="hidden" name="sectionId" value="3">
The value for sectionId can be determined by going to section settings in the control panel (example.com/admin/settings/sections) and clicking the name of the section you want to use Guest Entries with. The section’s ID is the last segment of the URL (e.g example.com/admin/settings/sections/3)
<input type="hidden" name="redirect" value="success">
The value for redirect is where a visitor will be redirected upon a successful submission and should be a valid path or URL.
A form field should be added for each field you want to save content to. Be sure to include any fields marked required on the entry type’s settings page (example.com/admin/settings/sections/3/entrytypes/3).
<input type="text" name="title">
title will always be the value for the name attribute for the entry’s title regardless of what Title Field Label is set to for the entry type in the control panel.
<textarea name="fields[body]"></textarea>
Custom fields will use fields[customFieldHandle] as the value for the name attribute.