This error results when:
1) you don't link to a Form Item View Page using a Reference Code.
Each submitted form's data is has a unique code that is used to reference the data when displayed on a Form Item View Page.
Here is an example of how to link to data from your Form List View Page's Layout area:
Click on 'Help' from within liveSite and search for "reference code" for all the details on setting up and using reference codes to access form data.
2) or you have created a Custom Form Page that specifies another Page to be sent that references a Custom Form too, but the two custom form names don't match. In other words, a Custom Form Page can send a Custom Form Confirmation Page when submitted, but the Custom Form Confirmation Page must specify the same Custom Form. (See E-mail Administrator or E-mail Submitter in Custom Form Page Properties).