Hey there!
Please have a look at this topic for more insights. But long story short, the model has some faulty surfaces, which are making it get flagged as a sheet body. Once the faulty parts are fixed and everything is a solid, you should be good to go!
Check this simplified model to see the difference: https://www.simscale.com/workbench/?pid=1295674389835736516&mi=geometry%3A1&mt=GEOMETRY