In some cases the Harmonize tab will not resolve cases with non-consecutive IDs, returning the error message: Consecutive IDs are not consecutive. This will happen if any of the land cover categories in your earlier or later map has a pixel counts of zero. The error can be resolved if only one of the land cover map contains categories with 0 pixel counts. The error cannot be addressed if both of the maps have different categories with 0 pixel counts.
To resolve, move any of the categories that have 0 pixel counts to the end of your legend. For example, in the Harmonize tab, change the code of those 0 pixel count categories to the maximum code.