Samba Share not showing up under "other locations" in Ubuntu 20.04

Ubuntu Server or Ubuntu Desktop being used as a server? If it is Ubuntu Server:

A modern samba server also "announces" itself to the network using mDNS ( Avahi ) so that machines running MacOS and Linux can also "discover" it. The problem here is that Ubuntu Server does not install avahi by default so you will have to install it:

sudo apt install avahi-daemon

You should also make sure it is installed on the client.

I suspect you will have another problem however with the Ubuntu 20.04 client. Because of a bug in the gvfs backend in Ubuntu it will try to access the server using the SMB1 protocol ( samba calls it NT1 ) which the Ubuntu server has disabled.

You can enable it on the server by adding the following line in the [global] section of smb.conf:

server min protocol = NT1

Then restart smbd:

sudo service smbd restart

Side note:

There is no such thing as only guests. It's only guest without that last "s". Samba is ignoring it which is fine since I don't think you need it.