Possible reason ...
Sometimes ADE gets its registration/activation confused and in a semi-authorized state.
Uninstalling and reinstalling does not help.
Unfortunately, it often then gives misleading error messages about what is wrong.
A common incorrect message informs you that the ID is already in use on another computer and cannot be reused.
This can often be resolved by completely removing any authorization using ctrl-shift-D to the Library screen on ADE (cmd-shift-D if on Mac).
Restart ADE, and then reauthorize with your (old) Adobe ID.
I have done that, but it didn't resolve my problem. I'm still getting the error.
I have reset my computer and since then, I have lost all my books in my library. I want to redownload the books in my Adobe digital app.
See (possibly not very satisfactory) answer in your other thread.