Can't test LinuxLite 7.6 in a virtual machine

[stevef]

Therefore, the latest updates in 7.4 contains the utilities to do a version upgrade, right?

Yes
When a new Linux Lite release is made, all existing versions (in the same series) get notified as part of the regular round of updates. After this flag has been set, the in-series upgrade becomes possible. If you didn't update your 7.4 system, it could not know that 7.6 is out so in-series upgrade is not available.

Thank you for your patience.

Prego.
We still don't know what is wrong with your VM install of the 7.6 iso.
As I don't know anything about VM, I'm probably not able to help on that.