DIAGNOSIS: That's the cooker telling you it doesn't think the lid is closed all the way. The C01 code is almost always a lid position or sensor issue.
FIX: This is usually a simple mechanical fix. First, unplug the unit. Open and close the lid a few times, firm but gentle. Check the silver hinge pin on the back—make sure it's seated correctly and not popped out. Then, look at the latch on the front. There's a small plastic tab inside the lid that needs to slot into the main body's latch. Make sure no food debris is blocking it, wipe it clean, and close it until you hear a distinct, solid click. Plug it back in and try.
IF THAT DIDN'T WORK: The sensor inside the lid hinge might be faulty or have a loose connection. If the lid looks perfect and clicks shut, but you still get C01, you'll likely need to contact Ninja support. It's a known failure point they can help with.
HEADS UP: Never force the lid shut or try to run it with something holding the latch down. That safety sensor is there for a reason—to prevent pressure or steam accidents.