I never really messed with the HumanoidDescription, but can’t you as well just change the decal object inside of the character which is the character’s face.
There are very heavy restrictions to HumanoidDescriptions and conventions you need to follow. Ones that this decal does not follow. I suggest you use the actual Joyful Smile and not someone else’s reupload for this reason, or ensure that your item follows conventions.
If you want to use a face decal with a HumanoidDescription, you must upload the decal to your own models. HumanoidDescriptions have asset usage ownership restrictions equivalent to InsertService. When uploading the face, make sure the decal is named face explicitly. Use that resulting uploaded ID.
You need to supply details about what you tried then if you are looking for help. I can’t be told that it doesn’t work without an full explanation as to what steps you took leading up to the issue and pictures to help the explanation.
You can try alternatives too, such as directly changing the face decal’s texture over using the HumanoidDescription face field for the time being while trying to figure out what went wrong in your implementation.