welcome to the forum!
Although vCenter 6.5 is not certified for vOneCloud I hope I can help you to know what is happening.
When do you see that error?
- When you try to get the list of templates that you want to import after clicking on Get Templates or
- When you try to import one of the templates on the list generated using the Import button?
If the first situation was implied (most probably), could you try to log in to the appliance as explained here and as a root try to do the following?:
- su - oneadmin
- Edit the /usr/lib/one/sunstone/routes/vcenter.rb file and go to to line number 103 and replace the line logger.error("[vCenter] " + e.message) with the following line logger.error("[vCenter] " + e.message + "/" + e.backtrace)
- Save the file
- Restart services and try to import the templates again. This time, hopefully, we would get the line number that complains about the undefined method `host'. With the line number in hand we can try to understand what may be happening and what else it's needed.