You’ll have to move on from the original Super Mario Maker if you want to create maps. Nintendo has warned that course uploads will no longer work for the Wii U game as of March 31st, 2021. If you want to craft a ridiculously difficult level or otherwise share courses with your friends, you’ll have to use Super Mario Maker 2 on the Switch.
Nintendo didn’t explain why it was winding things down. You can still play courses after the cutoff, but the game will clearly lose some value when you can’t share your creations with anyone else.