EDITAR
Acabei de notar que parece que você está tentando criar um link simbólico entre dois compartilhamentos de rede. Não me surpreende que isso não funcione corretamente. Por que você quer fazer uma coisa dessas? Eu recebi o mesmo erro quando testei também agora usando o Windows 10 ee dois servidores Linux Samba. Estou apostando que essa funcionalidade não é suportada, mas se for, talvez ela seja suportada apenas por servidores Windows. Eu não posso testar isso facilmente.
Qual sistema operacional executa seus servidores de armazenamento?
Resposta original
De acordo com este site, talvez seja necessário fazer algumas alterações de configuração para que isso funcione. O site tem melhor formatação, mas eu citei abaixo caso o site desapareça.
The mklink command also allows you to create a symbolic link targeting a Universal Naming Convention (UNC) path. For example, if you run the following command, Windows will create a symbolic link file called Link.txt that opens the Target.txt file. Mklink link.txt \server\folder\target.txt If you enable remote symbolic links (discussed later in this section), they can be used to store symbolic links on shared folders and automatically redirect multiple Windows network clients to a different file on the network. By default, you can use symbolic links only on local volumes. If you attempt to access a symbolic link located on a shared folder (regardless of the location of the target) or copy a symbolic link to a shared folder, you will receive an error. You can change this behavior by configuring the following Group Policy setting: Computer Configuration\Administrative Templates\System\NTFS File System\Selectively Allow The Evaluation Of A SymbolicLink When you enable this policy setting, you can select from four settings: Local Link To Local Target Enabled by default, this allows local symbolic links to targets on the local file system. Local Link To Remote Target Enabled by default, this allows local symbolic links to targets on shared folders. Remote Link To Remote Target Disabled by default, this allows remote symbolic links to remote targets on shared folders. Remote Link To Local Target Disabled by default, this allows remote symbolic links to remote targets on shared folders. Enabling remote links can introduce security vulnerabilities. For example, a malicious user can create a symbolic link on a shared folder that references an absolute path on the local computer. When a user attempts to access the symbolic link, he will actually be accessing a different file that might contain confidential information. In this way, a sophisticated attacker might be able to trick a user into compromising the confidentiality of a file on his local computer.
Também notei que você tem uma barra invertida em falta no caminho UNC, deve ser \fileserver1\files\test.pdf