Hi @전우진(WOOJIN JEON) 한화시스템/ICT,
Welcome to Microsoft Q&A!
Which version of Visual Studio 2019 did you install? Community, Professional or Enterprise?
For VS Community, it doesn't support offline activation via a product key. You must sign in to your Microsoft account occasionally in VS Community.
If you are using Professional or Enterprise version of VS, you can use a product key to activate your VS in an offline environment. To do that, run your VS as administrator > File > Account Settings > select Unlock with a Product Key. It is noted that prerelease versions of Visual Studio don't have product keys. You must sign in to the IDE to use prerelease versions.
Sincerely,
Anna
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