This is just a very common error when you trying to access the vCenter Server with PowerCLI and when you have an invalid server certificate, probably the first time you are trying to connect with PowerCLI. You can easily fix it by executing a line of command. I hope this will be really useful.
Connect-VIServer Connect-VIServer : 2/20/2019 6:27:50 AM Connect-VIServer Error: Invalid server certificate. Use Set-PowerCLIConfiguration to set the value for the InvalidCertificateAction option to Prompt if you’d like to connect once or to add a permanent exception for this server. Additional Information: Could not establish trust relationship for the SSL/TLS secure channel with authority ‘VCENTER_SERVER’. At line:1 char:1 + Connect-VIServer <VCENTER_IP_ADDRESS_FQDN> + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + CategoryInfo : SecurityError: (:) [Connect-VIServer], ViSecurityNegotiationException + FullyQualifiedErrorId : Client20_ConnectivityServiceImpl_Reconnect_CertificateError,VMware.VimAutomation.ViCore. Cmdlets.Commands.ConnectVIServer

Just execute below command to ignore the invalid certificate
Set-PowerCLIConfiguration -InvalidCertificateAction Ignore -Confirm:$false
I hope it helps.
Tom
May 31, 2019The solution does work if you connect to vCenter server once per PowerShell session. If you connect to the same vCenter again, it will fail with the same error message. So all scripts that use multiple connections to the same vCenter don’t work anymore and I have to open a new PowerShell session for every script I want to run. Worked fine with PowerCLI 6.5R1. The issue came with PowerCLI 10 and persists with version 11. I probably need to open a SR.
Aruna Lakmal
May 31, 2019Thanks Tom for your valuable feedback!
Vivek Joseph
April 8, 2020It helped, thanks a lot sir!!
Aruna Lakmal
April 15, 2020Thanks for the comment Vivek!