My first two guesses with the little info provided is either
you have built the form on a non-updateable query or you
have the form set not to allow additions. You can check the
latter in the properties of the form under Data.
Is your form based on a query? I'm seen this problem
before when a form was based on a query that was in turn
based on two or more tables. If there is no relationship
set between the two tables in the query, you will get this
error message.
Also, you might check to be sure your primary key is set
up correctly.