When using the vSphere Client it is only possible to export a virtual machine as a template in OVF-format. Exporting in OVA-format is not supported at the moment. This functionality has been removed since vSphere 6.5 (documentation).
It is however possible to export a virtual machine in the OVA-format with the VMware OVFtool via the command line.
Download the OVFtool from VMware and install it on your platform. In this example it was installed on a Windows machine.
Help for ovftool is available with the -h parameter, including many examples. Here is the command I used to export a machine from vCenter to an OVA-file:
ovftool.exe --noSSLVerify --targetType=OVA vi://administrator@vsphere.local@172.20.10.94/SA-Datacenter/vm/Lab-VMs/QA-CentOS "c:\temp\QA-Centos.ova"
In this command 172.20.10.94 is the vCenter server's IP-address. Note the vm part in the virtual machine path right after the datacenter name. You will not see this folder in the vCenter Server inventory but it is there nevertheless.
