Can you tell me in detail? How can do? I have no idea about this

I have Accounting software which is prepared in VB, SQL server. Actually when I delete one or both of the rows containing “InOutHistory” by using SQL Enterprise Manger with the following steps;
1. Right-click on the table.
2. Click on Open table, and then click on Return All Rows.
3. Highlight the rows, and then press the Delete button.
I received this message "Key column information is insufficient or incorrect....