That means one of dependencies of an add-on that you are trying to install is also listed in another repository that is pointing to an offline source.

The way to fix it is to disable old repositories that may have been abandoned or you’re not using them anymore.

In order to know which repository is the culprit, look at the Kodi error log. It should show you where it is trying to install a dependency that is failing.

How to Fix “Failed to Install a Dependency” Error in Kodi