Unit authorization of e-reader

A downloaded ebook OK to read on my lap top.

In my e-reader ("Letto") it is registered, but does not open up for reading.

Instead it informs that "a unit authorization is required to read adobe DRM publications.

a box marked "OK" ends the message. Clicking OK takes me back to the library of the reader.

Authorization of my lap top, where I download, has been done. How to authorize a reader for adobe DRM?