Module development and code questions

Hi,

I have added previous and next buttons in drupal 8 form. Here is a screenshot of my form

https://imgur.com/a/3FoWxi7

After clicking the 'Next Level' button, I want to select the next chapter from the Select Chapters fields.

How can that be done in forms? 

This is part of my code for chapters field:

Hello, 
I have a problem with the restws, I manage to recover data, but when I want to do an update I get an error: 406Not Acceptable : UuidEntityException: Trying to save a Content entity with empty or invalid UUID.
Please, could you tell me what the problem is?

I have a problem fews a day.

In this case I created service with login function response login is work but status code internal error 500

I call to services/session/token and get token attach in header then call test_api/user/login.json

X-CSRF-Token : token

Content-Type : application/json

after response is failed because status code 500 but response text is right.

I can't attach image capture. Thank you for read. apologize for my english language.

I have a Drupal 7 custom form which uses Drupal.behaviors to modify a select menu to dynamically alter a form. It works, except a form element that is exposed when submitted is hidden when form_set_error() is invoked.

This appears to be the only comment that gets close to what I'm looking for. https://www.drupal.org/forum/support/module-development-and-code-questio...

Hello, I have pages created such as privacy policy, instructions, etc that I have to edit manually in the frontend.

How can I create content in the Backend and read it in the frontend?

Could be possible to use Style and all HTML functions? 

Hi! Anyone among you knows if there is any module for job offers. The idea is to create a subpage on which job offers would be displayed.
At the top there would be a panel to search for a given job after entering the position we are interested in.

edit PL:

Hej! Wie ktoś z was czy jest jakiś moduł do ofert pracy. Chodzi o to, by zrobić podstronę na której byłyby wyświetlane oferty pracy. 
Na samej górze byłby panel do wyszukiwania danej oferty pracy po wpisaniu interesującego nas stanowiska.