The Calc program is split into many component files; components are loaded automatically as you use various commands that require them. Occasionally Calc may lose track of when a certain component is necessary; typically this means you will type a command and it won't work because some function you've never heard of was undefined.
If this happens, the easiest workaround is to type M-# L
(calc-load-everything) to force all the parts of Calc to be
loaded right away. This will cause Emacs to take up a lot more
memory than it would otherwise, but it's guaranteed to fix the problem.
If you seem to run into this problem no matter what you do, or if even the M-# L command crashes, Calc may have been improperly installed. See section Installation, for details of the installation process.