Hello jak.son48,
Thanks for that info and for choosing the Apple Support Communities. If we understand correctly, you are unable to use the Shake to Undo feature of your iPhone. If you haven't already, please restart your device as iOS clears cached data and does other system optimizations when you turn your device off and on.
Restart your iPhone
Next, make sure your iPhone is fully up to date, as the updates contain stability fixes that can resolve issues like yours, as well as important security updates to keep your iPhone safe.
Update the iOS software on your iPhone, iPad, or iPod touch
Update your carrier settings on your iPhone or iPad
Cheers!