Class scrlttr2 only supports foldmarks on the first page of the letter. They are put on the first page in the same way (using pseudo lengths) like the senders information, the address, the logo etc.
But you can load package scrlayer-scrpage and define new layers for the fold marks using their pseudo-lengths
\usepackage[manualmark]{scrlayer-scrpage}% sets automatically page style scrheadings
\DeclareNewLayer[%
background,
innermargin,
oddpage,
height=\useplength{tfoldmarkvpos}+.5\useplength{foldmarkthickness},
hoffset=\useplength{foldmarkhpos},
mode=picture,
contents=\putLL{\rule{\useplength{tfoldmarklength}}{\useplength{foldmarkthickness}}}
]{tfoldmark}
\DeclareNewLayer[%
clone=tfoldmark,
height=\useplength{bfoldmarkvpos}+.5\useplength{foldmarkthickness},
contents=\putLL{\rule{\useplength{bfoldmarklength}}{\useplength{foldmarkthickness}}}
]{bfoldmark}
\DeclareNewLayer[%
clone=tfoldmark,
height=.5\paperheight+.5\useplength{foldmarkthickness},
contents=\putLL{\rule{\useplength{pfoldmarklength}}{\useplength{foldmarkthickness}}}
]{pfoldmark}
Then you have to add the layers to the page style(s) used on subsequent pages:
\AddLayersToPageStyle{scrheadings}{tfoldmark,bfoldmark,pfoldmark}
%\AddLayersToPageStyle{plain.scrheadings}{tfoldmark,bfoldmark,pfoldmark}
Note that the first page of the letter uses page style empty.
MWE:
\documentclass{scrlttr2}
\usepackage{lipsum}% only for dummy text
\usepackage[manualmark]{scrlayer-scrpage}% sets automatically page style scrheadings
% declare new layers for the fold marks on subsequent pages using their pseudo-lengths
\DeclareNewLayer[%
background,
innermargin,
oddpage,
height=\useplength{tfoldmarkvpos}+.5\useplength{foldmarkthickness},
hoffset=\useplength{foldmarkhpos},
mode=picture,
contents=\putLL{\rule{\useplength{tfoldmarklength}}{\useplength{foldmarkthickness}}}
]{tfoldmark}
\DeclareNewLayer[%
clone=tfoldmark,
height=\useplength{bfoldmarkvpos}+.5\useplength{foldmarkthickness},
contents=\putLL{\rule{\useplength{bfoldmarklength}}{\useplength{foldmarkthickness}}}
]{bfoldmark}
\DeclareNewLayer[%
clone=tfoldmark,
height=.5\paperheight+.5\useplength{foldmarkthickness},
contents=\putLL{\rule{\useplength{pfoldmarklength}}{\useplength{foldmarkthickness}}}
]{pfoldmark}
% add the wanted fold marks to the layer page style(s) used on subsequent pages
\AddLayersToPageStyle{scrheadings}{tfoldmark,bfoldmark,pfoldmark}
%\AddLayersToPageStyle{plain.scrheadings}{tfoldmark,bfoldmark,pfoldmark}
\begin{document}
\begin{letter}{address}
\opening{Hello}
\lipsum
\closing{Bye}
\end{letter}
\end{document}

scrlttr2foldmarks. If you need additional help, then please add a MWE (minimal working example) to your question. – esdd Feb 20 '20 at 14:47