VS 2017 : The security debugging option is set but it requires the Visual Studio hosting process which is unavailable

This solved my issue:

Most likely, you have accidentally gotten the bit flipped to debug with ClickOnce security settings. Can you get the project properties for your app, go to the "Security" tab, and make sure to uncheck "Enable ClickOnce Security settings" or check the "This is a full trust application" radio button.

In case it helps anyone else - I have the same scenario - a multiple startup solution that includes a client that will be deployed with ClickOnce. To eliminate the problem that the client doesn't start after getting the Security Settings dialog, I moved it higher in the list in the startup projects dialog. If the client project is above the server project in the list, no error, everything debugs. If the client project is below the server project, then I get the error and the client never opens. This doesn't exactly SOLVE the problem but is a perfectly adequate workaround for me.

EDIT: You might need to close and reopen your Visual Studio for this workaround to be effective.

I spent hours trying to figure out the issue, this resolved it.

Go to Projct > Properties... > Build

Uncheck the checkbox Prefer 32-bit