The Appy Index Operator fails to run with the Sensor: Landsat 8 and Index: NDWI. Why?
The research article by Gao entitled “NDWI – A Normalized Difference Water Index for Remote Sensing of Vegetation Liquid Water from Space” references a NDWI index calculated as (NIR – MidRed) / NIR + MidRed). The article states the MidRed Channel used has a spectral Wevelength centred at 1.24. If you now refer to the USGS Landsat Information: https://pubs.usgs.gov/fs/2015/3081/fs20153081.pdf from Table 1. The L8 OLI/TIRS Sensor does not have a Mid-Infrared Channel, or a wavelength equal to 1.24.
This is the reason why the Indices Tool (from the Classification Toolbar) would hide the NDWI index for the Landsat Sensor. Likewise this Index within a Spatial Model cannot locate the desired band and will fail by default. However many have wanted to re-run this similar calculation but on a custom band selection as a pseudo' NDWI result. To do this, select ‘show all’ from the Indices tool then manually select the MidIR band you want used OR create a spatial model formulating the index manually and do not use the Apply Index Operator for this specific sensor and index as the default values are not valid and will need to be customized.
If using the Apply Index Operator and it fails in future - review you have the correct Sensor Selected AND a valid index available for the spectral bands available.