What specifically are you trying to do? Removing an item and deregistering a device are totally separate operations. Are you trying to deregister from the device itself? Which specific device and model do you have?
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You can always reach customer service for help here: amazon.com/contact-us
I am having the same trouble. I have 3 very very old kindle devices that I want to deregister and I get that error message. Tried chatting with amazon to fix it and they don't know what's wrong. Maybe they've gotten too big to provide good customer service anymore
I contacted Amazon through their customer service chat and they were able to delete my old devices. I just stated the issue very specifically and noted which screens I were on where I could see the devices. I also sent them a list of the devices by copying the names and the dates the devices were registered.