Simpletest fails with "invalid database prefix" - no other information given
For functional (browser) tests you need to provide some environment variables, in this case the SIMPLETEST_DB environment variable.

It can either be supplied in the phpunit.xml.dist file, or in phpunit command, or actually added to the environment before executing the phpunit command.

The format of the variable is seen in the phpunit.xml.dist file provided by Drupal core.

&lt;!-- Example SIMPLETEST_DB value: mysql://username:password@localhost/databasename#table_prefix --&gt;
&lt;env name="SIMPLETEST_DB" value=""/&gt;


From the error it seems that the provided SIMPLETEST_DB is not valid.

Note: the author mentioned that he was using the Admin UI and this answer does not address this problem.