What does it mean when you get this error message?

"the changes you requested have not been successful because they would create duplicate values in the primary key or relationship"

and what can you do to resolve it?

can anyone help please?

Without more information, it's difficult to know what you're doing wrong. From your error message, I'd guess that you are trying to create identical values for a column that is marked as the primary key. Primary keys must be unique.

I have a form with a linked subform and when i try to add to the subform that error message appears

The error means exactly what it says. You have some column or group of columns that you've defined as unique. So you can't put dupes into them. And you're trying to. How you would fix it is to filter out the duplicates from your insert or update statement that's throwing the error.