When I try using \preccurlyeq in math in lyx, and then compile I get error: Undefined control sequence
How can I fix that?
Go do Document > Settings > Math Options and then make sure "amssymb" is set to "Load automatically". If that doesn't work, change it to "Load always".
In the future (and also here if the above does not work), please always post a .lyx minimal example. For more information, read this: https://wiki.lyx.org/FAQ/MinimalExample