Fellow user here … I suspect there is a mismatch between the Roon Core version and the Roon Remote App version on your tablet.
(The Nucleus is shipped with an older build, normally the new Roon apps are able to connect and instigate the update, but it seems there is an issue with the current combination.)
This a link to the current build numbers …
If you go to http://nucleus in a browser, what does it show as the build number for “Roon Server”?
If you can’t reach it with http://nucleus, try http://nucleus.local, and if this does not work try http:// followed by the IP address of the Nucleus (which you should find on the list of connected network devices in the admin web page of your router).
This should display the Nucleus Web Administration Interface …
On that screen, you can see the Roon Server Build number …
Solution Proposal
I would recommend clicking on Operating System - Reinstall (This will reinstall both RoonOS and RoonServer).
This should bring the Nucleus fully up to date and then you should be able to connect to it via the Roon Remote App to continue your setup.
If you still have problems, connect a HDMI monitor to the Nucleus and post a screenshot of what it displays.