Apart from clicking on the 2 DoF(s) message to highlight elements in the sketch, you can simply drag different points and lines to see what can still move (nothing should be able to move). Then add missing constraints/dimensions. Here you may notice that two arcs (middle right and bottom right) are ...

Just select the slice and press Create body in Part Design. This will turn the slice into a BaseFeature within a Body, making it possible to work on it in Part Design.

Unfortunately, this constraint in FreeCAD FEM can be applied only to faces, not edges so you would just have to select the adjacent faces instead of the edge.

That's the problem with YouTube tutorials - they often show old versions. This panel was reworked and the Edit controls section was removed in 0.21.

Even a very small, invisible gap caused by finite precision of alignment could potentially cause this. That's also why boolean union often fails when the parts are just touching, not intersecting a bit (fuzzy boolean union with tolerance setting in the Defeaturing workbench can handle this but it's ...

Those lines are disconnected:


Quick and dirty fix using Draft Downgrade/Upgrade and Draft Move:

Jonas231 wrote: ↑Tue Apr 02, 2024 11:42 am How would I do the tie constraint?

It's described here: FEM_ConstraintTie

davidgunzenhauser wrote: ↑Tue Apr 02, 2024 10:22 am Does this mean I lost my work and need to start the project over?

Look for backup files (.FCBak). You can use them normally after changing their extension to .FCStd.

Apparently you saved the project to .FCStd file but with only the STL model (green icon) in the tree. Try it on a simple model - export to STL and save the FreeCAD file without opening the STL (so that you still see the modelling history in the tree).

It's not necessary to use both boolean fragments and compound. I would just make a compound. Then you can use tie constraint if something is disconnected.