This problem can happen on Google TV, which is different from Android TV (this is very confusing).
While Google TV is not officially supported, it can install Android TV apps and thus the Cloud Signage app can be installed.
However, sometimes sign-in with Google fails on Google TV devices (e.g., Chromecast with Google TV, TCL Google TV), showing a message like "Sorry, this app is not supported. Please try again later".
Currently, there is no fundamental solution. A possible solution to work it again is either of the followings:
Note that a factory reset clears all the data stored in a device and should be carefully conducted.
At the moment, we have to say that "Google TV" should be avoided when using the Cloud Signage app.
Official "Android TV" devices are highly recommended for the Cloud Signage app.
We are seeking for a complete solution together with Google developers and will update this page if it is found.