Hello @pravinyab -
Assuming that the land cover map is a raster, each of the 333,171 grid cells will have a value assigned, based on the 24 categories. Usually, land cover maps have an integer value assigned to each grid cell, so, for example, if you have 24 land use classes, they will correspond to integer values 1-24, where, say, “urban” areas have a value of 1, “agriculture” a value of 2, etc.
One very big problem with QGIS is that it does not allow you to see the raster attribute table, which would directly show you these integer values (and perhaps the class name, if the raster has them.) But if you use the GIS information tool and click around, you should see that each grid cell has a particular numeric value. The land cover map should have come with documentation showing which integer value corresponds with which land cover type (like 1=urban, 2=agriculture…) You can use this information to create the carbon pool table, providing these integer values in the “lucode” field. So you only need to create carbon pool data for 24 classes, not 333,171. The model requires that all values in the land cover map have “lucode” values in the carbon pool table.