When the skill type has no skills, you can still set shortcut keys, but an error will be reported。
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First issue is fixed and will be ready in next update of Hotbar plugin. For the second issue, it's likely due to your RPG Maker MZ isn't up to date. The latest version is 1.9.0
Update: Please update Hotbar Creator to ver 1.2.3 and it'll be fixed. The second issue you need to update your RPG Maker MZ.
https://sanghendrix.itch.io/hotbar-creator-rpg-maker-mv-mz-plugin/devlog/898297/...