Typically this can be solved by closing and re-opening the app. Sometimes waiting 1-2 minutes between attempts also helps. 

In some cases, this message appears in-place of the more specific device time and date error.

Another thing to try is logging in at https://endlesss.fm from your web browser, and after a successful login trying again in the app.

If this also doesn't solve it, please contact us at support@endlesss.fm and include the version number in your report. Also include whether this occurs on Endlesss Studio, Endlesss Mobile or both.