inline add not working for detail table

I have the exact same version and it is working perfectly fine for me.
https://phprconsultant.co.za/tutorials/jc/jobcard_master_list.php

Thank you Corrie,
I upgraded project from v_10.7 to 10.8 and after the upgrade the inline_add functionality is gone. But the existing 10.7 project still works good with inline_add. Is there anything that I can try to achieve this functionality for the upgraded version?
with regards.

Check for JS errors in the Developers Tools Console.
If you have an error in your event code, the page will act as you explained.

Hi Alexey, thank you so much for the tips. I see following error in the console while explanding the child/detail table. Will you please help me identify and fix this issue?

custom_functions.js?3522_1661087084:67 Uncaught TypeError: Cannot read properties of undefined (reading 'parents')
at Runner.pages.RunnerPage.init (custom_functions.js?3522_1661087084:67:45)
at constructor.init (RunnerAll.js?39712:1708:624)
at constructor.init (RunnerAll.js?39712:1730:1217)
at constructor.init (RunnerAll.js?39712:1801:1541)
at constructor.init (RunnerAll.js?39712:1900:136)
at constructor.goInitQueue (RunnerAll.js?39712:1374:305)
at constructor.<anonymous> (RunnerAll.js?39712:1374:102)
at constructor.<anonymous> (RunnerAll.js?39712:18:277)
at Runner.util.Event.fire (RunnerAll.js?39712:21:196)
at constructor.fireEvent (RunnerAll.js?39712:26:512)

You can find the code that gives you this error on the Events screen -> custom_functions.js

If you our help with it, contact support directly at https://support.xlinesoft.com or email to support@xlinesoft.com

@Alexey8, many thanks. You were correct, I edited the custom custom_functions.js file and things look good now. Appreciate your tips to have this issue resolved.