JSON error: data did not match any variant of untagged enum AppGroupRepr

Solution: Pass the group id instead of the group name as shown below:
"default_group": "xxx09123-xxxx-421xxxx-xxxxxx0564xxx1f0"
 
The whole request would look something like:

{
    "name": "NewApp",
    "description": "AppCreateTesting",
    "add_groups": {
    "xxxxxxxx-xxxx-xxxx-xxxxxxxxx": [ "ENCRYPT" ]
    }, 
    "default_group": "xxxxxxxx-xxxx-xxxx-xxxxxxxxx",
    "enabled": true,
"oauth_config": null
}