I guess you are trying to pass a tensor to your .format() call. Which is not supported.
In particular, BER looks like it is a Tensor of size 1, len(snr_t) so that might not be possible to print with format.
If you just want the one for this sample, I guess you want:
        BER = 1 - accuracy[0][k]
        print('({0}) BER:{1:6.6f}'.format(t + 1, BER.item()))