You probably originally got the book with one Adobe account/ID, and are now trying to read it on a compter/device authorized withj a different account/ID.
DRM books are locked to the Adobe account/ID used when they are first accessed.
You must register ADE with the same Adobe account you used before.
Best action to take depends on the exact reason you got into that situation.
Solution for one reason copied below ~~~ if your reason is different you may need different actions but the basic principle is still the same.
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DRM books are locked to the Adobe account/ID used when they are first accessed.
You must register ADE with the same Adobe account you used before.
To do that, first deregister the new account (ctrl-shift-D, cmd shift-D on Mac), then register the old account.
If that Adobe account is associated with an old email, you can change the associated email without changing the underlying basic Adobe account.
Log on at the Adobe website (using your old email), go to My Adboe, then to My Adobe ID, and change the email to you new email.
If you have created a new account/ID with your new email, then you will have to change that one first to something you never use;
then you can change the email for the old account/ID.
