That indicates that you are trying to go from a previous anonymous activation to a 'named' (with an AdobeID) activation.
ADE tries to do this by associating your current anonymous (Adobe ebook DRM) account with the AdobeID. What the error message is saying however is that your AdobeID already has an Adobe ebook DRM account associated with it (ie it's already being used), so it can't use that.
Your choices are:
1.) Deactivate the computer and then when you relaunch ADE authorize with your AdobeID. IMPORTANT NOTE: this will mean that you will no longer be able to read any of your current content that is authorized to your current anonymous account.
2.) Create a new AdobeID and authorize with that new AdobeID. IMPORTANT NOTE: this means that if you have purchased content (on a different computer) with your current AdobeID that you have to manage the two libraries seperately.