By default, ESXi only allows signed vSphere Installation Bundle (VIB), because an unsigned VIB represents untested code installed on an ESXi host. You can change the acceptance level for each host, in the Configure, System, Security Profile menu, under the Host Image Profile Acceptance Level option:

The host image profile supports the four acceptance levels:

• VMware Certified: VIBs created, tested, and signed by VMware.
• VMware Accepted: VIBs created by a VMware partner but tested and signed by VMware.
• Partner Supported: VIBs created, tested, and signed by a certified VMware partner.
• Community Supported: VIBs that have not been tested by VMware or a VMware partner. Community Supported VIBs are not supported and do not have a digital signature. To protect the security and integrity of your ESXi hosts, do not allow unsigned Community Supported VIBs to be installed on your hosts.