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#Racket, 58 bytes

(λ(l)(andmap(λ(m)(andmap(λ(n)(not(member(+ n m)l)))l))l))

Explanation:

(λ(l)(andmap(λ(m)(andmap(λ(n)(not(memq(+ n m)l)))l))l))
(λ(l)                                                 ) # Define a lambda function that takes in one parameter
     (andmap(λ(m)                                  )l)  # If for all m in l
                 (andmap(λ(n)                   )l)     # If for all n in l
                             (not(memq(+ n m)l))        # n + m is not in l