This reason behind this error message. This device is activated with an Anonymous account which has a unique UUID stored in the registry. When you try to activate the same machine with different Adobe ID (UUID), this error message will be thrown.
Solution1: Deactivate the anonymous account and activate with the existing Adobe ID
Note: In this case, the books you have fulfilled with the Anonymous account will be permanently lost.
If you like to continue reading the books fulfilled by Anonymous account, you have to follow solution 2
Solution 2: You have to assign the UUID of anonymous ID to newly created Adobe ID. For more information please refer to, Adobe Digital Editions – Anonymous Activation
Hope this solves your issue.
How do I deactivate the anonymous account in Adobe digital editions? And how do I then activate my own Adobe ID?