Looks like appvolume is unable to connect to vcenter (machine Managers) verify the same and if requried edit and add again.
Next Go to Infrastructure TAb and verify the Storage Tab is listing the Datastores availale in the vC.
Then go the configuration tab and edit you will have the option to set the default storage option.
I ran into this issue with my customer today. I tried to re-enter the vCenter login credential again without success. I even upload the default template again and still no luck.
My customer was using the SQL always on and there is something wrong with the replication sync.
I had to create another App volume DB and re-install App volume manager and everything was working fine.
Re-scan and import the appstacks again and set the user entitlement.
On the machine manager tab, I confirmed (re-entered) the password. Then went to the storage tab and the data stores were visible.
Thanks for the replies.
Good to know its solved.