@tourette95, the exporting (to Excel) was designed to export one Excel sheet with one NxN connectivity matrix. In your case, these metrics are time-resolved (windowed) and frequency-resolved (spectral) metrics, thus, there is an NxN matrix for each window and for each frequency point. As @Marc.Lalancette mentioned, this feature is not supported in Brainstorm yet.
As such, for the moment, if you want to export the a time-resolved spectral connectivity metric, it is necessary first to extract one point in time (process: Extract > Extract time), and the use the Export to file indicating which frequency is the one to be exported.