The memoir class enforces the so-called “new” font changing commands (although they have now been in use for
many years.) This means that it does not allow you to use the commands \bf, \it, \rm, etc.
Unfortunately there are some old packages that still use these commands, and there is also some
outdated information on the web that might have examples using these commands. You should
generally not use these commands (see Two Letter Font Commands); if you do encounter problems,
you can pass the option [oldfontcommands] to the class.