Building off the base of the first game, Mario + Rabbids Sparks of Hope piles on a ton of extra things to do in each world. There are battles, of course, but also a host of things to do and find in each of the overworlds. From side quests to finding Sparks, Planet Keys, and even Secret Zone challenges, it will take a good deal of effort to complete each one. Thankfully, the game provides a handy percentage meter, so you can see how close you are to doing everything there is in a given world.
However, even after getting all the Sparks, Planet Keys, memories, and quests, you may still find yourself stuck at 99% completion and not know why. This isn't a bug, but the game won't tell you what that last percentage point is related to. If you're confused as to why you can't hit 100% completion in a world in Mario + Rabbids Sparks of Hope, here's the last thing you need to do.
How to get 100% completion
After you've done everything the game asks you to in a given world in Mario + Rabbids Sparks of Hope and find yourself only at 99% completion, there's one task left to do. Again, the game will never tell you what it is, but it is related to the Secret Zone challenge.
Step 1: Return to a world you've at least completed the Secret Zone challenge in.
Step 2: Access the Secret Zone challenge a second time to begin it again, this time with a time limit.
Step 3: Beat the Secret Zone challenge within the time limit to unlock a special weapon skin.
Step 4: This will reward you with the final 1% you need to complete the world.
Step 5: Repeat for all five Secret Zone challenges.