Softplus currently does not have a quantized implementation.  This error also usually means that a quantized tensor is being passed to a non-quantized function.
For a quick fix, you could add a torch.quantization.DeQuantStub() and torch.quantization.QuantStub() around the areas of the network which cannot be quantized such as Softplus.
The longer term fix would be to add quantization support for Softplus to PyTorch.