The font size command \normalsize is not defined

The LaTeX error:

! LaTeX Error: The font size command \normalsize is not defined:
               there is probably something wrong with the class file.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...
 
l.3 \begin{document}
                    
?

reports something pretty fundamental (document base font size has not been set, something the document class does for you).

It can, in principle, be a problem with the document class, but is more often caused by the user forgetting to start their document with a \documentclass command.

FAQ ID: Q-normalszmiss