Hi @ja_ba_bu: You can help yourself. For that kind of internal simulation you need just one solid. Currently you have 4 solids.
If in your CAD software you make a boolean ‘union’, you should get just one solid. Importing it to SimScale will open the door to correct meshing and simulation.
Another way would be to click on your geometry in SimScale and select ‘Add geometry operation’. You should merge your four solids into one, becoming a simulation domain (just the water inside your device).
Take care,
Retsam