Oh no! That shouldn’t happen! It looks like you got a mix of code from the old and new MFiX versions. The solver now lives in anaconda3\envs|mfix-23.3\lib\site-packages\mfix_solver not the Scripts directory as indicated in the error popup above.
Please submit a full bug report - main menu - Submit bug report in the main menu - and upload the ZIP file here. Thanks.
Another workaround is to build a custom solver in the project directory, with SMP and DMP disabled - the mesher is looking for a non-parallel solver and if one is found in the project directory it will be used.