STL files should be able to be uploaded. However, I’ve also encountered problems with STL geometries myself and prefer to work in other formats, namely IGES.
While I’m unaware of a method to fix the STL file such that it is able to upload, there are several ways to covert the STL file to a more friendly format. Maybe someone here can have a fix for the STL upload but in the meantime, I will detail out how to convert the STL file.
The first method is the way I usually use. However do note you need AutoDesk’s AutoCAD for this method. Convert your STL file to DWG via this link, then import it into AutoCAD before exporting the model as IGES for upload. Be sure to select the IGES format when uploading.
Another method is to use Onshape, but Onshape has similar issues with STL files from what I’ve experienced. So you can try to use the previous link to convert to DWG and upload it to Onshape to then export it to SimScale. I haven’t personally tried this method so if it works do let me know. An easier way since you don’t need any additional CAD software other than Onshape which is in the browser.
The last method is to mesh the geometry via a third party mesher and upload that to SimScale. This saves you from needing to upload the geometry in the first place but you need third party software to manipulate and mesh the geometry which is less than ideal.
Hope this helps!