This error happens because the input files in the Process1 were none or wrong type. In fact the process process_extract_scout should have been grayed out.
The Extract > Scout time series process needs source files as inputs: 
or 
If you are a new Brainstorm user, we recommend you start by following the introduction tutorials first (section "Get started" on the tutorial page), using the example dataset provided.
https://neuroimage.usc.edu/brainstorm/Tutorials