To use the SLIDINGAGG Function 

SLIDINGAGG(Tag1,OPERATOR,WINDOW SIZE, WINDOW OFFSET)

SLIDINGAGG(Demo-influxdb/DEMO_01TI031.PV,MAX,12h)

Give max value of this tag in a rolling 12h window

Configure

When you select SLIDINGAGG this is what will come up

You can see there is no function to this (Sliding AGG of what?)

An operator needs to be added

The easiest way to to this is to add it in at the top line then press enter

SLIDINGAGG(0,OPERATOR,0,0)

Valid Operators