Can you select those faces and create a sketch on them.
If not, they are not a flat face.
If you can you could then generate a sketch to give you your needed output.
Otherwise you can create a plane perpendicular to a curved surface and then a sketch on that.
What I did was to add a new offset construction plane. Then project onto it which worked fine for the front, rear & bottom, but I am unable to create an offset construction plane for either of the sides. I get the same error message.
Below is the tool you want to use to add the plane.
After completing the sketch, I selected it in the panel and ‘moved’ it away to show it.
Using the project tool also works, after adding the parallel plane.
Add it, move it away, select the face and project.
Thank you. I tried that and it worked perfectly.