Thank you for trying FEATool. Please provide as much information about your Matlab installation, system configuriation and how to reproduce the error if possible, and use the correct subforum with a proper question title. This will help us to help you, and also help others in the future.
Your error message indicates that there is some error in your FEATool installation, more specifically either the included lib/mex/assemble_s.mexw64 file or corresponding Matlab path is missing.
FSI SOLVER IS NOT WORKING. THROWING ERROR MSG. assemble_s.mexw64 not found.