Cannot access MySQL database. File missing: /etc/mysql/root-defaults-ubos.cnf

This is known to happen under some circumstances if you run UBOS from a very slow disk, such as a USB 2.0 stick. During startup of the database engine (MySQL/MariaDB) required by certain Apps, a timeout may have occurred that prevents the database initialization from completing.

You can try to start the database manually with:

% sudo systemctl start mysqld.service

and then attempt to re-install your App. If this does not work, use a faster disk: For one, if your disk is too slow to successfully install an App, chances are it is also too slow to do anything useful with the App.