Hello,
Export of vm as OVF from ESXi 6.7 host client (UI client) fails and it's a known issue as per VMware Support. VMware Engineering team is working on it & will be fixed shortly.
You can achieve this goal from vCenter webclient & it Works like a champ.
You can use vCenter webclient to export OVF till the time ESXi host client is getting fixed.
Thanks!