Let $n > 13$ be a positive integer. Is there any $n \times n$ circulant $\pm1$ matrix $A$ satisfying the following property $$AA^T=(n-1)I+J$$ where $I$ is the $n \times n$ identity matrix and $J$ is the $n\times n$ matrix of ones? I conjecture that the answer is no. But I can't prove it.