It is probably one of Janome’s many blessings. Do you have the straight stitch needle plate on? Or the HP plate?
The plate you have on determines which stitches become available as a preventive to breaking a needle. The machine won’t let you do a zig zag because the needle will strike the plate...and break!
This is usually the answer to your dilemma.
How many times was I just as aggravated as you? More than I care to admit. 🤣
I always hope the “wizards” at the other end of a web query always tell the truth; but for now, I ask those mysterious people everything from “How much mustard is a substitute for dry mustard”, to “Why won’t my Janome sew a zig zag?”