It's a legality checker, and it tells you what is invalid, not a possible way to fix things.
There isn't a silver bullet recommendation for every single thing that can be wrong with a Pokemon. Pick an invalid IV spread? If it's invalid, pick a different one that the encounter is able to have. Changed the species, but now it's invalid? Go verify what it takes to evolve it, and make sure all the signs of it doing that are evident in the data. There are many evolution methods, like contest stats (milotic), level up, trade. It would slow the program down if it has to determine a suggested fix every time you checked legality, and it would slow down general development of the program if the developer has to automate every single suggestion for lazy users who change values without care.