If you are using Debian stable release, wordpress might be obsolete and doesn’t work well. Perhaps, you can’t upgrade any themes or plugins. You need to install the latest package from testing or sid release.

Following steps might be helpful.
1. add apt source line.
deb http:///debian {testing|sid} main contrib non-free
2. edit or make /etc/apt/preferences.d/wordpress and insert followings.
Package: wordpress wordpress-l10n php-getid3
Pin: release n={testing|sid}
Pin-Priority: 101
3. apt update
4. apt install wordpress/{testing|sid} wordpress-l10n/{testing|sid}
5. go to /var/lib/wordpress/wp-content
There are some directories such as plugins, languages or themes.
These directories contain some symbolic links. You must delete all of them and copy the same contents from /usr/share/wordpress/wp-content/*.
6. Change these files owner and group, due to upgrade contents and plugins from Dashboard.
chown -R www-data:www-data /var/lib/wordpress/wp-content/*

I’m so happy if you clear your problems after read this blog.
Thanks!