I have TV14 installed (under 10.14.6). I tried to upgrade it to TV15 by the dedicate function but, after downloading and starting the re-launch process, it gets stuck. I waited some minutes invane.
So, I tried to install TV15 directly but... surprise: after launching the installation and after accepting the license, it shows NO DISKS where to install! That's why, I think, also the upgrad from within TV14 does not succeed.
So, I tried:
1) to delete the app file from Applications (but not all the other preferences, fearing to loose a lot of customizations). But
2) instead, I tried to install using another User Account of the same Mac (obviously, an administrator) so that this user is clean. But it happens the same: no disk is shown and so the process hangs indefinitely.