Windows 11’s default security policy forces SMB signing and disables outdated cryptographic protocols like TLS 1.0, which ESXi 5.5 relies on.
If the vCenter is offline or you need a fresh install, you can try to find it in the official download portal, though navigating the new Broadcom portal can be challenging: vsphere client 5.5 download for windows 11
While you can technically download and force the vSphere Client 5.5 to run on Windows 11 using compatibility tweaks, it is highly unstable. If you are still running ESXi 5.5, your primary goal should be migrating to a supported version (like vSphere 7.0 or 8.0) that utilizes modern, browser-based HTML5 management. Windows 11’s default security policy forces SMB signing
Managing bare-metal ESXi 5.5 environments with the "C# thick client" is highly discouraged for modern workflows due to operating system security vulnerabilities. How to install vSphere 5.5 client on a domain controller Managing bare-metal ESXi 5