Look at the error message
'ASP.webform1_aspx' does not contain a definition for 'GridView1_SelectedIndexChanged'
What is is looking for a method called GridView1_SelectedIndexChanged? Look at your gridview definition
onselectedindexchanged="GridView1_SelectedIndexChanged"
Ok, now look ate your code-behind, do you have a method called GridView1_SelectedIndexChanged? No, so it doesn't know what to do when the selected index changed event is fired.
If you are interested in this event you need to add this event-handler to your code-behind, if you're not interested in the event remove the relevant attribute form your gridview definition.
GridView.SelectedIndexChanged Event (System.Web.UI.WebControls)[
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