Hi Michael,
Auto Mananger is a community tool which is not a product of Dynamics, which is based on the auto number feature of Dynamics CE.
https://docs.microsoft.com/en-us/dynamics365/customerengagement/on-premises/developer/create-auto-number-attributes

From Dynamics CE perspective, you can try to create a auto number attribute easily with web API like this using Tabbed Postman tool, and see if everything is fine.
-----------
POST [Organization URI]/api/data/v9.0/EntityDefinitions(LogicalName='new_widget')/Attributes HTTP/1.1
Accept: application/json
Content-Type: application/json; charset=utf-8
OData-MaxVersion: 4.0
OData-Version: 4.0
{
"AttributeType": "String",
"AttributeTypeName": {
"Value": "StringType"
},
"Description": {
"@odata.type": "Microsoft.Dynamics.CRM.Label",
"LocalizedLabels": [
{
"@odata.type": "Microsoft.Dynamics.CRM.LocalizedLabel",
"Label": "Serial Number of the widget.",
"LanguageCode": 1033
}
]
},
"DisplayName": {
"@odata.type": "Microsoft.Dynamics.CRM.Label",
"LocalizedLabels": [
{
"@odata.type": "Microsoft.Dynamics.CRM.LocalizedLabel",
"Label": "Serial Number",
"LanguageCode": 1033
}
]
},
"RequiredLevel": {
"Value": "None",
"CanBeChanged": true,
"ManagedPropertyLogicalName": "canmodifyrequirementlevelsettings"
},
"SchemaName": "new_SerialNumber",
"AutoNumberFormat": "WID-{SEQNUM:5}-{RANDSTRING:6}-{DATETIMEUTC:yyyyMMddhhmmss}",
"@odata.type": "Microsoft.Dynamics.CRM.StringAttributeMetadata",
"FormatName": {
"Value": "Text"
},
"MaxLength": 100
}
Or you can post your query to Auto Number Manager community : https://www.xrmtoolbox.com/plugins/Rappen.XrmToolBox.AutoNumManager/