This error may appear in the following cases:

1. If you transfer an improved pack to a character who does not have a main pack.
2. If you transfer multiple packs to one character simultaneously.
3. If you transfer a pack to a character that already has any other pack.
4. You can't pass any pack to the same character twice.

For example, you are trying to send improvement to character B, but the main set was previously transferred to character A. Or transfer two sets to one character at a time.

However, if you are still unable to transfer the pack to your character, please contact Support. 

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How do I apply the contents of a pack?
Pack contents are automatically assigned to your account. Account-wide items (such as CBT access and Premium Account time) will benefit all your active characters for a specified duration. Single use or permanent features intended for a character you create (such as armour, titles and kits) will be deposited in a special web-inventory from where they can be assigned to an active character.

Once assigned, the item or feature cannot be removed and given to another character.

If you subsequently delete a character to which you have assigned something, the item or feature will simultaneously be lost.