Validate deployment failed deny policy in SQL managed instance Azure
Figured out the solution:

Go to the resource group you want to create the managed instance in > Policies > Assignments > Allowed resource types (with the scope subscription/resourcegroupname)   parameters > then from the allowed resource types dropdown select Microsoft.Sql.managedInstances

Sign out and into your account, then managed instance will deploy.

When I got the 'Validate Deployment - 'deny' Policy action' error I went to the Activity log, clicked the Validate Deployment error dropown, clicked the 'deny' Policy action, then looked at the JSON tab - if I looked sequntially to see where the error originated I found this section

"category": {
    "value": "Policy",
    "localizedValue": "Policy"
},
"eventTimestamp": "2019-10-30T13:04:54.8516173Z",
"id": "/subscriptions/XXXXXXX/resourceGroups/XXXXXXXXXXXXX/providers/Microsoft.Sql/managedInstances/minamehere/events/77999a0f-d551-45e0-9b3b-1156b246b50d/ticks/637080374948516173",


"resourceId": "/subscriptions/XXXXXXXXX/resourceGroups/XXXXXXX/providers/Microsoft.Sql/managedInstances/minamehere",
    "status": {
        "value": "Failed",
        "localizedValue": "Failed"

Which is what led me to policies and allowed me to troubleshoot and fix the issue - somewhere I knew I needed to add 'Microsoft.Sql/managedInstances' to the Policy category, which fits with the deny policy error I was experiencing.