Nintendo currently carrying out maintenance to fix a bug that occurs in “Create” mode and has taken the Super Mario Maker servers offline. Until the updates are complete, online services for the game will be unavailable.
Nintendo is in the midst of preparing a software update to fix this bug. The online service will be available again after the software update is distributed.
This maintenance is expected to be completed by tomorrow evening.
Latest network status and maintenance info, is available on the Nintendo Network Maintenance information page.