I think System.DirectoryServices requires an manual reference to System.DirectoryServices.dll which can be done just by right clicking your project in solution explorer -> Add Reference -> in .NET Tab,
Now, you should be able to import System.DirectoryServies.AccountManagement namespace.
If you still can't see it like its shown in above picture then you should try adding reference like this,
<add assembly="System.DirectoryServices.AccountManagement, Version=18.104.22.168, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
It doesn't matter if you are using application page or not, I would recommend you to go through this article for a better understanding of this new class for Directory services.
Hope it helps.