There's a problem with 20.04 accessing NAS share.
You can check if you have an old Ubuntu 18-19 live dvd: it can access NAS share out of the box without tweaking anything.
In 18-19 and 20.04, only samba.lib, cifs.lib, nfs.lib are installed. It's sufficient in 18-19 but it does not work for NAS share in 20.04.
So the solutions commonly found (full cifs, full samba, full nfs) all miss the point: you did not need to do any tweaking in 18-19.
Smb.conf is installed if you install FULL samba for directory sharing (other than NAS) for instance and it works well in 20.04. In Gnome 3.36, if you share a folder, it installs full samba automaticaly and it works out of the box in 20.04. But you won't be able to acccess NAS shares or if you can the objects (avi, jpg, text etc.) are all seen as folders.
This behaviour is a known bug suppposedly recently solved. (Page bottom)