I didn't mention:

The sketch mirror tool operates on a Closed sketch, not on a Open sketch. Select the desired sketch to mirror and click the mirror tool.

Of course, you'll want to turn on the sketch visibility = true, or you won't see the closed sketch.

Without your file, I am unable to help resolve the Boolean error.

Try copying the bad boolean file (paste somewhere).

With the bad bool gone, see what happens.

Then, in a new file, open the bad bool and investigate...

Click the sketch, Edit>Copy (or duplicate). (I don't keep the ref's when prompted) Paste the sketch into another Body. Then, open the Sketch workbench and use the sketch mirror tool to flip it. If tool not present, learn how to add the tool/icon to the toolbar. FYI - I do things my way: I think of a...

First, verify all is good by doing a simple test.
Attached are the three individual part step files and the assembly step - go thru the process I laid out and see what happens...
Then, Also attached is the Freecad file without the boolean op

There are many ways to accomplish a Modeling goal. I prefer using PartDesign workbench 95% of the time. To the show-n-tell of my post: I created the footprint and extruded a solid. Then, drafted the sides, cut the top off part of it to get the lower elevation section, added radius’… Then, made a qui...

Having come to FreeCAD from SolidWorks, ProEngineer, Inventor... other, using PartDesign with it's arrangement of sketches and Pad/Cut/Etc, similarity between those other programs is maintained for sensible design process - if you know one, you basically know the others including FreeCAD/PartDesign....

Having come to FreeCAD from SolidWorks, I've done this a bazillion times. Here's the scoop: An Assembly was exported from SW as a Step file. So, import all the files into FreeCAD. Open FreeCAD, Open the PartDesign Workbench and Create a new File and a Part. (the Yellow icon) Also create a Body insid...

I suspect by end of day, you'll have 10 different approaches...

This one is a simple, quick example only. No intention of doing it like I would do it if it were my project but, you'll get the point.

Mirroring it was done in Part wb (as "bejent" indicated)...

It would be great to have configurable placeholders... That's easily done - I just did it with a Macro created by 'Recording the steps' Record Macro, Select the Body/Part/Whatever, File>Export, Select output file type, Enter a filename for output file, Select the directory, Apply Stop Recording. Pl...

For me, Exporting for 3D Printing happens magically, perhaps because I use a Mac. The export panel opens with the filetype of (whatever) the last file type I exported. Screenshot1. I always Export STL. So, it opens with stl as the filetype. If I export an OBJ, then next time, it auto selects OBJ. Ca...