When we use OnClienClick script for a button in asp.net, you can notice that all the associated validation controls (validation group) will fail to validate. This happens because the OnClientClick script overrides form validation script.
To fix this, here is the work around.Add the below code in Page Load event
Button1.Attributes.Add("onclick", "if(confirm('do this ?') == false) return false;");
This will execute the client script and then executes the form validations scripts before post back.
Hope this helps