Hey @AbdulAzizM,
This is happening because you are trying to export the vApp completely and import it to a standalone ESXi. The vApp feature only exists in vCenter Server, having said that if you do not have this ESXi connected to vCenter, it is going to fail.
For importing the VM into the standalone ESXi you will need to export each VM individually as an OVF template and import it one by one by first unregistering from vCloud without deleting the VM.