When I had this issue first setting up the integration, it was due to devices in LibreNMS and Oxidized were identified by different address types. One had IPs and the other had hostnames. Cannot recall which was which though.
Also, the device types need to be mapped, for example, ciscowlc is aireos, juniper is junos etc.
When does this error appear for you?
Is it when you test the return of groups in the API query by running the command "curl -H 'X-Auth-Token: YOURAPITOKENHERE'https://librenms.org/api/v0/oxidized"?