SOLVED: I have found the reason. It was Polylang plugin. I couldn’t assign different laguages to the tags, so my slag had the same name in all the languages. It is necessary to create a different slug for different languages For example: Tag (in English) -> tag Tag (in French) -> tag-fr
For fixing Polylang not showing + option in the tags menu see this: https://wordpress.org/support/topic/plugin-polylang-categories-and-tags-cannot-assign-language?replies=8
