Sitecore 9.0 is a welcome update to the Sitecore developer community. Although using the Experience Editor or the Content Editor is preferable at times, us developers love using Sitecore Rocks. From Sitecore 9.0, connecting with Sitecore Rocks became harder with its new permission issues.
This issue can be reproduced by following the steps shown below:
On Sitecore Explorer in Visual Studio, do a Right Click on Connections then press New Connection.
Type in your connection settings then press Test...
You may get this error if your Web.config is not configured properly.
To fix this issue,
Open C:\inetpub\wwwroot\tac.corporate<Name for your installation>\Web.config on your favourite text editor.
<allow users="?,*" />
Add the highlighted lines after </system.webServer>.
You should get a positive test result after this change.