Can’t find “Allow Add New” option for relationship field related to taxonomy

Hi,

I can see in your documentation about relationship fields that there is an “Allow Add New” option “compatible with relationships with Post Types and Taxonomies.

However, I have relationship fields related to taxonomies, but I don’t see any “Allow Add New” checkbox available.

I would like to add new entries directly from the Post Type I am in, without switching to the taxonomy, and then go back to the Post Type and reload the page. That would be much handier.

So, is there a reason I can’t find this option?

Thank you in advance for your help.