You can’t access this shared folder because your organization’s security policies block unauthenticated guest access.

Not sure which option eventually worked for me, but one of these eventually worked for me:

Local Group Policy -> Computer Configuration -> Administrative Templates -> Network -> Lanman Workstation -> Enable insecure guest logons.

using Powershell:

Set-SmbClientConfiguration -EnableInsecureGuestLogons $true -Force

Set-SmbClientConfiguration -RequireSecuritySignature $false -Force

Set-SmbServerConfiguration -RequireSecuritySignature $false -Force

Source: https://www.reddit.com/r/WindowsHelp/comments/1htk1jp/you_cant_access_this_shared_folder_because_your/