score:2

Accepted answer

Short answer: use axis=1 in concatenation layer.

Long answer: The output of Conv3DTranspose layer (i.e. output5 tensor) has a shape of (None, 1, 199, 199, 3). Now, you want to concatenate 5 copies of this tensor (which sounds a bit odd to me since I don't know the application, but surely you know what you are doing!) on the second axis. Therefore, you need to use axis=1 (and not 0 which would be the batch/samples axis) as the concatenation axis to get an output of shape (None, 5, 199, 199, 3), i.e. 5 copies concatenated together.