Have you changed your Wato rule during development? Because this error normally means that you have configured a rule with a rule definition that does no longer fit to the current definition. This means you have to delete and recreate this rule or just change it to fit the current definition.
Regarding service discovery: The device does not need to have it’s own enterprise branch. You only need any SNMP value that can be used to identify the device. E.g. you can use some text in oid .1.3.6.1.2.1.1.1.0 to trigger the scan.