Forena Reports, does not appear in the Drupal menu Structure I installed the downloaded the 8.x-1.x-dev version of the Forena module, and it installs correctly. When I go to the menu Structure to start creating the report, the Forena Reports link doesn't appear.
How can I solve this problem?
I also have the Forena UI sub-module enabled, but Forena's Release Notes include this:

Basic rendering of reports and document types are complete, however the GUI version of the report writer has not yet been ported. Sufficed to say, this version can be used by Drupal 8 report writers who are comfortable writing .frx files and sql elements by hand.
This is, pretty sure, a permission issue. So go check if the user has the relevant permissions granted. If not that's why the link is not shown either.
Some more debugging hints:

just enter relative URL /reports and see what happens next (it may be available, even though the menu item is still missing for some reason).
temporary logon with user/1 (to bypass any potential permission issues).

BTW, the GUI version of the Forena report writer is just a user interface to facilitate the design (construction) of Forena report specifications (saved in so called FRX files). So for sure that is not related in any way to your missing menu item, which if shown lists the available reports that you can execute, e.g. the sample reports.
Documentation and examples are included in the module you installed / enabled. For an illustration of that, still D7, refer to this online demo, which also includes:

Some report samples.
Forena Documentation, such as a Setup Guide (with more details about things such as Permissions to be configured) and more details about the WYSIWYG Report editor.

Disclosure: I'm a co-maintainer of (the D7 version of) this module.