https://techcommunity.microsoft.com/t5/storage-at-microsoft/smb-signing-and-guest-authentication/ba-p/3846679

Single Independent PC:
Set-SmbClientConfiguration -RequireSecuritySignature $false


Domain-based group policy (on IT-managed fleets):

Locate the security policy applying this setting to your Windows devices (you can use GPRESULT /H on a client to generate a resultant set of policy report to show which group policy is requiring SMB signing.
In GPMC.MSC, change the 
Computer Configuration > Policies > Windows Settings > Security Settings> Local Policies > Security Options.
Set Microsoft network client: Digitally sign communications (always) to Disabled.
Apply the updated policy to Windows devices needing guest access over SMB.