Custom Field Won't Add

Dear WordPress friends,

This is my first time posting a troubleshooting and usually I get my problem done by only reading others’.

But I have searched over and there is a thread about this also. And… The solution did not work for me.

So I have been running the site for some months, still with the same theme. I could add the custom field ‘Thumbnail’ properly back then but I dont know what happened that the custom field wont add correctly.
When I press ‘Add Custom Field’ The yellow highlight won’t appear and it doesnt add properly.

I tried to remove all blank lines in function.php but nothing happened. Still the same.

Please help, I really want to have thumbnail images in my posts. Thankyou.

My blog: https://www.irenewithlove.com