It is grayed when there is no data available for correction.
Other possibilities:
- use manual
vignetting correctionlocated underprofiled lens correction. Do not use “vignette filter” which is for artistic purpose. - use Flat field correction
- help to update lensfun database see for instance http://lensfun.sourceforge.net/calibration/ or https://discuss.pixls.us/t/create-lens-calibration-data-for-lensfun/9848
- use LCP if it exists for your camera: see http://rawpedia.rawtherapee.com/Lens/Geometry
Simplest way to extract a LCP (or a DCP)
- download Adobe DNG converter
- change the suffix from .exe to .zip
- unzip preferably with 7-zip
- lcp are located in \DNGConverter_11_2_1.zip$APPDATA\Adobe\CameraRaw\LensProfiles\1.0\