You need to be on a Members server, in a bank spot, and must meet all the Completionist Cape requirements. If a new requirement came out recently (I believe some content was turned into a mini-quest recently?), you will need to complete it before you can Keepsake your Completionist Cape. Likewise, once Keepsaked, you won't be able to activate the cape until all new requirements have been met.@Lisaa_RS - Followed by @Runescape
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With the completionist cape rework earlier this year you unlock the cape as an override as well as an item when you purchase one from the Varrock museum. Hence, there is no need to use a keepsake key on the cape as you should already have a override available in the wardrobe.