You registered on the forum more than a year ago, but since it's your first post here, welcome.
The problem with this model is that it simply cannot be unfolded, there is a sharp corner on one side that I have highlighted in green:
One other thing that makes it impossible to unfold is the vertical bend at the rear of the part, its top edge is co-planar with the under-face of the main part so the faces are fused together.
Here's what I think: this is actually an item made of two separate sheet metal parts, but because no gap was added between them, it got exported as a single, fused part.
I can explain how to separate both parts and I'm pretty sure it will then be possible to unfold them. But maybe you should check with the person who sent you the STEP file if this is supposed to be two separate parts or a single one. In the latter case, they need to redesign it.
Plastec835 wrote: ↑
Wed Jan 02, 2019 4:15 pm
OS: Windows 10
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Your version is slightly outdated, the last version with bug fixes is 0.17.13541
. On the other hand, we are expecting for v0.18 to be released very soon.
Edit: fix typo