I can provide specific or troubleshooting steps based on your setup.

ESXi 5.5 follows a "Try and Buy" model. When you first install the software, it operates in for 60 days. During this window, every feature is unlocked. Once the 60 days expire, you must apply a valid license key to continue managing your virtual machines (VMs). The Free License vs. Paid Licenses

If you previously owned a license but cannot find the key, you have a few options:

It may not recognize NVMe drives or the latest Intel/AMD processor architectures.

The software is vulnerable to modern exploits.