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I already tried to flip and the same error happens. and the sketch is terminating at the body, although it doens't look like it...
Shorten the sketch entity so that its ends float in space. a virtual extension of each end should run into the solid body. Make sure that there is just a single body to terminate the rib. Depending upon the sketch orientation, you might need to toggle the parallel to sketch or Normal to sketch options. There may also be an ambiguity with the lower end of the sketch (lower right of your pic) because it appears to exactly encounter the curved edge. Once you apply thickness to the sketched line, the curved edge will no longer terminate the rib. Try moving the lower right endpoint to the left so that the extended sketch line will see the cylindrical face rather than the edge.
Rib feature uses the thin feature functionality. But it also extends open sketch ends until they encounter matter. It also applies a thickness. It also uses Up to Next end condition. When your sketch line is being extended, it hits the edge. So far so good. When the width is applied and the extrusion "fills" in the entire area, the portion I have outlined in red tries to extend until it is stopped by matter (up to next) but it would continue on to infinity and that's one good reason for a failure. In the above pic, I have used a simple thin feature extrude, not a rib feature to show you the difference between a simple thin extrude and a rib.
The below pic shows a typical failure mode of a rib:
You could follow my lead here and do this manually with two features: 1) a simple extrude as I did. and 2) two extruded sketched on the small remaining faces with converted edges and an up to surface end condition:
If this doesn't work for you, then we need to work with the part directly. Or submit it to your VAR in a support case.
Thx, Ben
I messed around with the rib command. Changed a few things around. I tried the shapes like you have a rectangle on the left with a round extension on the right. If I move my sketch to the outer limits it fails if I move to slightly inside it works with the round extension. I made the right extension a square and move the sketch to the outer limits and it works fine. My assumption is the rib does not like the round shape when the sketch is at outer limits.
Round extension, line slightly offset to inside
Square extension sketch to outer limits and works.
That's what I was trying to show Kelvin, But you described it way better than me. Nice explanation!
Your material side arrow indicator is pointing out to space. Toggle the Flip Material Side switch so that the arrow points to the solid body.
Also, know that the extensions of your sketched entity must terminate at the body.