The message may concern the reservoir's fitted position or recognition, not simply the visible water level. Check only the owner-accessible fitting procedure in the manual.
What the observation means
A container that is not fully seated can look correctly filled while the appliance cannot use it. Do not push it forcefully, pour water into surrounding openings or bypass a warning. Use the specified water type and fill limit.
A useful next step
If the normal fitting procedure does not clear the message, record it and the reservoir's condition. Do not keep restarting a steam programme to draw water through a suspected fault or open internal supply parts.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.