Re: "Error! Check activation"... What should I do?

If you can't erase authorization from ADE directly, you can follow the other steps given in the link:

https://helpx.adobe.com/digital-editions/kb/error-e_act_not_ready.html

Close the ADE first.

Now

For mac, you will have to manually remove the file activation.dat from "/Users/Library/Application Support/Adobe/Digital Editions" folder.

For Win, You will need to remove the registry. Steps for which are mentioned in the link.

I hope it helps.

-Chirag