The smb-protocols nmap script checks to see which smb dialects are present on the Samba server.

client min protocol sets the minimum smb dialect that server can use to connect to another server not the minimum dialects that a client to this server can use.

By default the version of samba used in Ubuntu 18.04 sets the server min to NT1 ( smbv1 ) and the max to SMBv3.

If you want to restrict access to this server to SMB3 and SMB3 only you need to set the server min protocol:

server min protocol = SMB3

*Note: there are sub-variants to this dialect:

SMB3_00: Windows 8

SMB3_02: Windows 8.1

SMB3_10: early Windows 10

SMB3_11: Windows 10

By default SMB3 in smb.conf selects the SMB3_11 variant.*

From the man page for the configuration file for the Samba suite in the "client max protocol" section:

Possible values are :

• CORE: Earliest version. No concept of user names.
• COREPLUS: Slight improvements on CORE for efficiency.
• LANMAN1: First modern version of the protocol. Long filename support.
• LANMAN2: Updates to Lanman1 protocol.
• NT1: Current up to date version of the protocol. Used by Windows NT. Known as CIFS.
• SMB2: Re-implementation of the SMB protocol. Used by Windows Vista and later versions of Windows. SMB2 has sub protocols available.
• SMB2_02: The earliest SMB2 version.
• SMB2_10: Windows 7 SMB2 version.
• SMB2_22: Early Windows 8 SMB2 version.
• SMB2_24: Windows 8 beta SMB2 version.

By default SMB2 selects the SMB2_10 variant.

• SMB3: The same as SMB2. Used by Windows 8. SMB3 has sub protocols available.
• SMB3_00: Windows 8 SMB3 version. (mostly the same as SMB2_24)
• SMB3_02: Windows 8.1 SMB3 version.
• SMB3_10: early Windows 10 technical preview SMB3 version.
• SMB3_11: Windows 10 technical preview SMB3 version (maybe final).

By default SMB3 selects the SMB3_11 variant.

Normally this option should not be set as the automatic negotiation phase in the SMB protocol takes care of choosing the appropriate protocol.

The value default refers to SMB3_11.

Adding clarification from A.B in the comments:

wiki.samba.org/index.php/Samba3/SMB2 states "Samba 3.6 added basic support for SMB2.0", "Basic support for SMB 2.1 was added in Samba 4.0.0", "Basic support for SMB3 is included in Samba 4.0.0 and later."

My testing with Nessus has indicated SMBv1 is only disabled when setting

min protocol = SMB2

in the [global] section of smb.conf . Core, LANMAN2 and NT1 were all still flagged as being vulnerable.

First, there is no such thing as a HomeGroup in Linux, MacOS, or Windows starting with Win10.

Second, if you are talking about that specific error when running smbclient you would need to enable SMB1 ( Samba calls it NT1 ) on the client side in /etc/samba/smb.conf. Right under the workgroup = WORKGROUP line add this one:

client min protocol = NT1

Of course the server you are trying to connect to must have SMB1 enabled on its side. If you kept your Win10 machine up to date it is disabled so you will have to enable it there. If the server is running Ubuntu 20 or anything with the same version of samba you would have to edit smb.conf on that machine and add:

server min protocol = NT1

Then restart samba:

sudo service smbd restart
sudo service nmbd restart

If you don't do that on the server you will likely get a protocol negotiation failed: error.

That will probably eliminate the SMB1 error message. Whether you can "discover" all the workgroups and hosts in Nautilus is another matter. If you have to turn the clock back to SMB1 days you need to follow the rules set at that era ... name resolve order, host name length restrictions, etc ...