error/moodle/detectedbrokenplugin

A broken plugin, such as a module, has been detected. In order for the upgrade to proceed, the broken plugin should be deleted.

In the case of a Theme, then it is more than likely there is a file that has not been renamed correctly, or the theme is missing a vital component like a version.php, or that the version.php is not correct in some small detail, usually the plugin version.