The message smells like you might be using Microsoft Access to edit a table stored in Microsoft SQL, but that you don't have all of the defined key columns in the edit control. If that is correct:
1. Verify that there is a defined key
1a. Should be a PRIMARY KEY definition
1b. Could be a UNIQUE constraint
1c. Might be a UNIQUE INDEX
2. Identify all columns in all of the potential defined key(s) from step 1.
3. Ensure that all of the columns from at least one of the defined keys are present in your edit control.
Nope, it isn't possible to "copy and paste a row in my table" because there isn't any GUI representation for a table. What you are doing (very possibly without realizing it) is copying and pasting within some application running on your machine, not actually on the table itself.
Since you haven't given us (the readers) many clues, but you have ruled out Microsoft Access, I'll make another guess... This time I'll guess Captain Peacock, in the Drawing Room... Oh wait, wrong game!
Let's try this guess: Are you possibly using SQL Enterprise Manager, and Editing a table grid control? If so, please let me know and we can start to solve your problem... If not, I might try to guess again!
Just in case you didn't get the idea, I'm frustrated. You're asking me to help you solve a problem, but you haven't given me anything concrete to work from. I assume that you are using SQL Server, and that you haven't used it very much so you're handicapped in explaining your problem, and it appears that English isn't your native language which would further complicate things for you... I'm trying to help, but I need you to help me more so that I can help you!
The problem stems from the fact that you do not have a unique primary key defined on your table. Thus, when you try to delete one record SQL Server cannot tell which record it is.
Add a primary key to your table. A surrogate key will do fine. But I also agree that you should not edit data from the Enterprise Manager GUI. Use Query Analyzer.
If it's not practically useful, then it's practically useless.