Edit button not working in dataview

Hello, I have a short question about my dataview. I made a connection to SQL server. Placed a dataview on my .aspx and turned on "edit, delete and insert". Delete and Insert are working fine, but when I "edit" I get the message: "The data source control failed to execute the update command"

What am I doing wrong Can anyone help me with this

Thanks in advance, Kea Klaassen