MacBook Air M1 doesn't charge after macOS Sequoia 15.4 update with “Battery is not charging” error

Open Battery Settings ... does it indicate "Battery Not Charging" or "Charging on Hold" in small lettering at the top of the Settings window and show something like 70%, 80% charge level? This can also be seen on the Battery menu item at the top of the screen of the Mac.

If this is what you see, that is normal behavior as the Mac does this to try to maximize the battery life. Allowing it to go to 100% all the time shortens battery life. It is better off staying at 50%-80% if connected to a charger all the time. The choice of 80% is a compromise between maximizing the battery life, but still keeping sufficient charge to use off the charger for a while.

Back on Battery Settings, where it says Battery Health, click the little "I" inside the circle, do you see Optimized Battery Charging and is it active? If yes, that is what your Mac is doing, this is normal behavior to make your battery last as long as possible before it has to be replaced (which is fairly expensive so it's best to delay that with these conservation techniques).