No non-CDS connectors have been created

What causes this issue?

CDS-based connectors are always available, without special configuration
steps. Other connectors - type "Common Runtime" - have to be created
individually per client.
Here, no connectors have yet been individually created, although there
is model content available. It can be assumed that "Common
Runtime"-connectors, too, are relevant in this system.

System Response

Searching is possible only for CDS-based connectors, which are always
present without manual activities.

How to fix this error?

If the system is meant to be run in a pure CDS-based mode, this is fine
and the warning can be ignored.
Use task list (transaction STC01) SAP_ESH_CREATE_INDEX_SC to create
connectors.
You can also start the ES-Cockpit (transaction ESH_COCKPIT), and there
manually create connectors.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.