I downloaded the zip file, unzipped it to the mod directory. Then as the site admin navigated to Site administration->Notifications.
The notifications page showed:
mod/skillsoft skillsoft Non-standard (about to be installed)
I clicked the Upgrade button at the bottom of the page and received the indicated error messages.
Plugin "mod_skillsoft" is defective or outdated, can not continue, sorry.
More information about this error
Debug info: Plugin is not compatible with Moodle 2.x or later.
•line 456 of /lib/upgradelib.php: plugin_defective_exception thrown
•line 265 of /lib/upgradelib.php: call to upgrade_plugins_modules()
•line 1421 of /lib/upgradelib.php: call to upgrade_plugins()
•line 311 of /admin/index.php: call to upgrade_noncore()