Error on form submition

Hi,
I'm new to zoho creator, and I'm getting an error with my first application. When I submit the form while the associate liste has 1 row, I got the error message saying:
1 . [t_29987000000004009] null, message from server: "Duplicate entry '' for key 2".

All my required fields are filled, and I remove any unique field.
Thx for your help
bidon1