In AddLandCover operator there is an External Files button […], can Corine Land Cover 2018 100m GeoTif be used instead of CCILandCover-2015?
From section 2.1 of user manual:
Land cover map is used to set parameters which are not easily retrievable from optical images, e.g. vegetation height. This is done using a look-up table (LUT) containing the parameter values for each land cover class. The LUT provided with Sen-ET plugin assumes a land cover map with a legend based on the UN Land Cover Classification System (LCCS). ESA’s Climate Change Initiative land cover map utilises this legend and is available in SNAP using the AddLandCover operator. Users are able to use other land cover maps more suitable to their area of interest (e.g. Corine for European applications) but in that case they are required to create the LUT corresponding to the legend of the map which they are using. This can be easily done by creating or modifying a text file.
This LUT is used to create maps of vegetation structural parameters. See section 22.214.171.124 of the manual.