The error message is related to ADM license usage, 'Tools->Licensing->ADM Licensing Consumed'.
It counts devices associated with services, 'Resources->Service Instances'. To clean up immediately you should either delete or disassociate services from devices.
You can determine if you have the ADM license module under Tools > Licensing
The way to clear this message is to navigate under Resources -> Service Instances in Device42 and then click on the "Device" column, this will sort them by devices, based on that you can check which device+service needs to be deleted, put its name under the search tab in the same page, Enter then select all and delete, this is the simplest way as we don't have a filter by devices names on that page.
This essentially deletes the services on devices and clears up the message.
Alternatively, you can use either autoclean rules https://docs.device42.com/tools/auto-clean-feature/ to clear service instances.