Dears,
I followed up the procedure below
https://kifarunix.com/install-and-co...-ubuntu-20-04/

and have created the proper account and shared path on Ubuntu 20.04 server. But now on Windows client machine, I've got the rejection to copy files into the shared path (see the attached screenshot). How to identify the reason of the problem?

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First, you double-check your Samba configuration for that particular share, paying particular attention to permission settings. Then, you may also get some clues by checking Samba's log files. You may have to increase the log level to see anything relevant.

One common problem is related to the protocol versions used by the Windows client and the Samba server. They may use different, incompatible versions. Check which version your Samba server is configured to use. My server at home stopped working with Windows PCs until I added these lines to the global section of smb.conf:
The internet has plenty of resources for determining Samba versions and setting them so that clients and servers can communicate. I don't remember which page told me to set the protocol level to 2.