Which version of Windows is not suitable for the Vault Server according to standard requirements?

The Vault Server is a critical component of the CyberArk solution and requires a specific server-level operating system to function properly. Windows 10, being a client operating system, is designed for personal use rather than enterprise-level server applications. Therefore, it does not meet the requirements for running the Vault Server effectively.

In contrast, Windows Server 2012 R2, Windows Server 2016, and Windows Server 2019 are all server operating systems specifically designed to handle enterprise workloads, including the security and performance needs critical for running CyberArk's Vault Server. These versions provide the necessary features, stability, and security protocols required for a secure environment managing sensitive credentials.

Utilizing a client OS like Windows 10 for a server role can lead to performance issues, security vulnerabilities, and a lack of support for the enterprise functionalities needed by the Vault Server, thus making it unsuitable for this purpose.