Cannot change ownership of 'public_html'

Hello, In order to fix a 500 error on my website, I tried to change the owner of all folders and files in public_html. I followed what the user asend said here : and so I did this command : chown -R my_user:my_user public_html
After doing this, I had to do the following command : chown my_user:nobody public_html
But I had the following error : chown: changing ownership of 'public_html': Operation not permitted
I do not understand because the previous chown command worked. And now I have a 403 error instead of a 500 error.. Can someone help me please? I'm french, sorry for my mistakes.