Oval File Missing

I really would appreciate a ova installation file. AFAIK the only way for using vmm on Synology.

There are tools to convert between different VM formats. On many systems you can perform a basic setup on a physical machine & then convert the physical to a VM.

I assume you are referring to Hassio? I know of no VM images for other installation options.

If you clicked their OVA link you would have found your answer:

Currently we had remove the ova files and publish a vmdk virtual disk, until we have better OVF template to generate our OVA. This VMDK work with (maybe you need convert the disk):