Your "problem"* is very likely in the Fan:ZoneExhaust, especially at the Balanced Exhaust Fraction Schedule Name. Read carefully the documentation for this field.

I say "problem" as in the cause of the warning. Whether it's actually a problem or not depends on whether you also have an AirLoopHVAC serving the zone.

Let's assume you have two zones:

• Zone A: the zone where air is being transferred from (the source)
• Zone B: the zone with the exhaust fan, that receives transfer air from Zone A.

If you are setting a Zone:Mixing object to transfer the entire airflow from zone A to B that is being extracted in the zone B with the exhaust fan, then you can set this field to a schedule of always 1.

Note that having a Zone:Mixing object will not affect the heat balance of the source zone (Zone A). You probably want to add another object in that zone, for eg a virtual Fan:ZoneExhaust that would consume no power but limit the return air to the AirLoopHVAC serving zone A by the same amount that is being transfered to zone B. Or, if it's not served by an AirLoopHVAC, you probably need a ZoneInfiltration:DesignFlowRate.

Bottom line: You are responsible for balancing the airflows yourself, E+ won't do it for you (at least outside of an AirflowNetwork implementation)