Make sure that inherit attribute points to the correct class. It generally happens when you copied a page from an existing page in the same project.
From the source file, it is clear that there are no buttons with name you have asked in the question.
I am not sure but you may give a try by changing the inherits property slightly (may b by adding just a '_' character at the last) in both the design and code page.
In case this doesn't resolve your problem, please let me know.
Update
Try pasting below event in your code page
protected void Button6_Click(object sender, EventArgs e)
{
}