Did you download the stencils from Internet? If yes, please make sure the language of the stencil is Korean.
Besides, I would suggest you go to File > Options > Advanced > Scroll down to the bottom, click "File Locations" under General, check the path of My Shapes.
General it should be C:\Users\<User Name>\Documents\My Shapes
.
Locate to My Shapes folder and , C:\Program Files or Program Files (x86)\Microsoft Office\root\Office16\Visio Content\1033
. Make sure they have same files with .vssx format via copying and pasting.
If the steps above do not work, please go to Control Panel (Large icons View) > Indexing Options > Advanced > File Types tab > At the bottom, select "Index Properties and File Contents" > OK > Rebuild the Index. Please wait some minutes until the process is complete.
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