DIAGNOSIS The C1 error is a communication fault between the main control board and the pressure lid sensor. It means the cooker can't read if the lid is properly sealed or not.
FIX Unplug the unit. Let it sit for 5 minutes. Clean the gold contact pins on the underside of the lid hinge and the corresponding sockets on the main body with a dry cotton swab. Plug it back in, ensure the lid is fully closed and locked. Cycle the lid lock mechanism a few times.
IF THAT DIDN'T WORK The pressure sensor itself in the lid may have failed. You'll need to contact Ninja customer support for a lid replacement under warranty. Do not attempt to open the lid housing.
HEADS UP Never use water or cleaner on the contact pins. A damp swab can cause corrosion and make the error permanent.