This gives clear indications to the player about what is going wrong for the most commonly seen issues.
A Training Bracelet is blocking the Downgrade.
The player doesn't have enough of the Key Item (message appears if what they already have is >= what is in the chest, most likely they have level 1 and only 1 Key item in the chest).
The player doesn't have enough Upgrade Points in the chest to apply the Downgrades they are requesting.
The inventory to hold the downgrade recipe is missing.
This gives clear indications to the player about what is going wrong for the most commonly seen issues.