Error_undefined_old_font_command

Class scrartcl Error: undefined old font command `…'.

Sometimes documents result in an error message like:

! Class scrartcl Error: undefined old font command `\rm'.

See the scrartcl class documentation for explanation.
Type  H <return>  for immediate help.
 ...                                              

l.4 This is {\rm
                  roman}

Instead of \rm, \sf, \tt, \bf, \it, \sl, \sc, or \sfb may be specified. Except for \sfb, these are font switching commands that have been defined in LaTeX 2.09. For LaTeX2ε these have been replaced by the so-called LaTeX Font Selection Scheme 2 (NFSS2). They are therefore no longer included in the LaTeX kernel since 1994 and are generally considered deprecated. Some classes, for example the standard classes, explicitly define these commands themselves. \sfb was an extension defined in Script 2.0 for bold sans-serif type. Because the old commands compared to the new toggles \rmfamily, \sffamily, \ttfamily, \bfseries, \itshape, \slshape, \scshape or text commands \textrm{... }, \textsf{...}, \texttt{...}, \textbf{...}, \textit{...}, \textsl{...}, \textsc{...}, respectively the corresponding math commands had some disadvantages, they were initially defined in KOMA-Script 2 only to ease the transition from Script 2.0 to KOMA-Script. Min. from 2003, however, these commands were already no longer documented in the KOMA-Script manuals. Since 2013, an explicit warning was issued when using these commands. Since KOMA-Script 3.20 said error is output instead.

If you use an editor that can also display the help for an error message or if you call the help yourself during an interactive LaTeX run (this is the default setting of the editor TeXworks) by entering an h, you will also get the message:

You should note that since 1994 LaTeX2e provides a new font selection scheme
called NFSS2 with several new, combinable font commands. KOMA-Script
classes had defined the old font commands like `\rm' only for compatibility
with old LaTeX 2.09 document styles of Script 2.0. Nevertheless, these
commands are deprecated and undocumented at least since 2003. Since 2013
KOMA-Script classes warned about soon removement of these deprecated commands.
Now, after two decades of LaTeX2e and NFSS2, these commands will not work any
more. If loading a package results in this error message, you should contact
the author of that package and ask him to replace the deprecated font command
`\rm', e.g., by `\normalfont \rmfamily `. Otherwise you should reconfigure
or replace the package. If you have used the old font command `\rm' yourself,
you should replace it, e.g., by `\normalfont \rmfamily '.
To make it work for now, you can use the already also deprecated class option
`enabledeprecatedfontcommands'.

In addition to the history mentioned above, this note contains both information on how the corresponding command should be replaced and a note on how to reactivate the old commands by using another option, quasi as a last rescue. However, this option is also considered deprecated. It can therefore only be used as a last rescue for a limited time. It is strongly recommended not to use the old commands anymore. Packages which use these commands and do not even define them themselves when required should also be considered critically in the opinion of the KOMA-Script author.