Thanks for posting in Q&A platform.
Based on provided information, my understanding is Windows 10 client cannot access to the shared folder on Windows NT 4.0. Please understand that we do not have such Windows NT 4.0 device for testing in our lab.
I would suggest you to try if the following methods could help you.
A. Please run the following command in PowerShell to doble confirm if the SMB 1.0 was enabled on win 10 client.
Get-WindowsOptionalFeature –Online –FeatureName SMB1Protocol
B. If the SMB 1.0 was enabled, please try the following steps on Windows 10:
Open Local Group Policy Editor-->Security Settings-->Local Policies-->Security Options--> find Network Security: LAN Manager authentication level and right click it and select Properties-->select Send LM&NTLM - use NTLMv2 session security if negotiated--> click Apply and OK
And then please kindly check if the shared folder can be accessed.
C. If the issue still persist, please try to add the user that needs access to the shared folder in the following Group Policy from windows NT 4.0 device side.
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