xfce4-appmenu-plugin%EF%BC%8Cmate could not be found.

This usually happens when an application is new in the repositories (or Flatpak) and isn't yet known to the Community website.

You can ask the Linux Mint team to import xfce4-appmenu-plugin%EF%BC%8Cmate into the Community website, by sending an email to root@linuxmint.com.