Visual studio 2010 can not debug when user identified information configed in web.config

I am using visual studio 2020 to do the web application development,and our application require high authenciation,so we add the user identified information in the web.config like this:

<identity userName="username" password="pass" impersonate="true"/>

However,after add these words,I found that I can not go to the debug model.

When I hit the F5,I will get the error message like this:

Error 7 Could not load file or assembly 'Microsoft.CSharp, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies......。 (Exception from HRESULT: 0x80070542)

If I remove the line in the web.config,I can debug again.

What is the matter?