An aromatic species has planar structure, conjugated $\mathrm{C}=\mathrm{C}$ double bonds, cyclic structure with all ring carbon atoms with $\mathrm{sp}^2$-hybridization and having a total of $(4 n+2) \pi$ electrons. Cycloheptatriene has a total of six $\pi$ - electrons but one ring carbon is $\mathrm{sp}^3$ - hybridized that restricts the conjugated movement of $\pi$-electrons. Thus, it is not aromatic.
